Btrfs: fix assertion failure when freeing block groups at close_ctree()

Btrfs: fix assertion failure when freeing block groups at close_ctree()

At close_ctree() we free the block groups and then only after we wait for
any running worker kthreads to finish and shutdown the workqueues. This
behaviour is racy and it triggers an assertion failure when freeing block
groups because while we are doing it we can have for example a block group
caching kthread running, and in that case the block group's reference
count can still be greater than 1 by the time we assert its reference count
is 1, leading to an assertion failure:

[19041.198004] assertion failed: atomic_read(&block_group->count) == 1, file: fs/btrfs/extent-tree.c, line: 9799
[19041.200584] ------------[ cut here ]------------
[19041.201692] kernel BUG at fs/btrfs/ctree.h:3418!
[19041.202830] invalid opcode: 0000 [#1] PREEMPT SMP
[19041.203929] Modules linked in: btrfs xor raid6_pq dm_flakey dm_mod crc32c_generic ppdev sg psmouse acpi_cpufreq pcspkr parport_pc evdev tpm_tis parport tpm_tis_core i2c_piix4 i2c_core tpm serio_raw processor button loop autofs4 ext4 crc16 jbd2 mbcache sr_mod cdrom sd_mod ata_generic virtio_scsi ata_piix virtio_pci libata virtio_ring virtio e1000 scsi_mod floppy [last unloaded: btrfs]
[19041.208082] CPU: 6 PID: 29051 Comm: umount Not tainted 4.9.0-rc7-btrfs-next-36+ #1
[19041.208082] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.1-0-gb3ef39f-prebuilt.qemu-project.org 04/01/2014
[19041.208082] task: ffff88015f028980 task.stack: ffffc9000ad34000
[19041.208082] RIP: 0010:[<ffffffffa03e319e>]  [<ffffffffa03e319e>] assfail.constprop.41+0x1c/0x1e [btrfs]
[19041.208082] RSP: 0018:ffffc9000ad37d60  EFLAGS: 00010286
[19041.208082] RAX: 0000000000000061 RBX: ffff88015ecb4000 RCX: 0000000000000001
[19041.208082] RDX: ffff88023f392fb8 RSI: ffffffff817ef7ba RDI: 00000000ffffffff
[19041.208082] RBP: ffffc9000ad37d60 R08: 0000000000000001 R09: 0000000000000000
[19041.208082] R10: ffffc9000ad37cb0 R11: ffffffff82f2b66d R12: ffff88023431d170
[19041.208082] R13: ffff88015ecb40c0 R14: ffff88023431d000 R15: ffff88015ecb4100
[19041.208082] FS:  00007f44f3d42840(0000) GS:ffff88023f380000(0000) knlGS:0000000000000000
[19041.208082] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[19041.208082] CR2: 00007f65d623b000 CR3: 00000002166f2000 CR4: 00000000000006e0
[19041.208082] Stack:
[19041.208082]  ffffc9000ad37d98 ffffffffa035989f ffff88015ecb4000 ffff88015ecb5630
[19041.208082]  ffff88014f6be000 0000000000000000 00007ffcf0ba6a10 ffffc9000ad37df8
[19041.208082]  ffffffffa0368cd4 ffff88014e9658e0 ffffc9000ad37e08 ffffffff811a634d
[19041.208082] Call Trace:
[19041.208082]  [<ffffffffa035989f>] btrfs_free_block_groups+0x17f/0x392 [btrfs]
[19041.208082]  [<ffffffffa0368cd4>] close_ctree+0x1c5/0x2e1 [btrfs]
[19041.208082]  [<ffffffff811a634d>] ? evict_inodes+0x132/0x141
[19041.208082]  [<ffffffffa034356d>] btrfs_put_super+0x15/0x17 [btrfs]
[19041.208082]  [<ffffffff8118fc32>] generic_shutdown_super+0x6a/0xeb
[19041.208082]  [<ffffffff8119004f>] kill_anon_super+0x12/0x1c
[19041.208082]  [<ffffffffa0343370>] btrfs_kill_super+0x16/0x21 [btrfs]
[19041.208082]  [<ffffffff8118fad1>] deactivate_locked_super+0x3b/0x68
[19041.208082]  [<ffffffff8118fb34>] deactivate_super+0x36/0x39
[19041.208082]  [<ffffffff811a9946>] cleanup_mnt+0x58/0x76
[19041.208082]  [<ffffffff811a99a2>] __cleanup_mnt+0x12/0x14
[19041.208082]  [<ffffffff81071573>] task_work_run+0x6f/0x95
[19041.208082]  [<ffffffff81001897>] prepare_exit_to_usermode+0xa3/0xc1
[19041.208082]  [<ffffffff81001a23>] syscall_return_slowpath+0x16e/0x1d2
[19041.208082]  [<ffffffff814c607d>] entry_SYSCALL_64_fastpath+0xab/0xad
[19041.208082] Code: c7 ae a0 3e a0 48 89 e5 e8 4e 74 d4 e0 0f 0b 55 89 f1 48 c7 c2 0b a4 3e a0 48 89 fe 48 c7 c7 a4 a6 3e a0 48 89 e5 e8 30 74 d4 e0 <0f> 0b 55 31 d2 48 89 e5 e8 d5 b9 f7 ff 5d c3 48 63 f6 55 31 c9
[19041.208082] RIP  [<ffffffffa03e319e>] assfail.constprop.41+0x1c/0x1e [btrfs]
[19041.208082]  RSP <ffffc9000ad37d60>
[19041.279264] ---[ end trace 23330586f16f064d ]---

This started happening as of kernel 4.8, since commit f3bca8028bd9
("Btrfs: add ASSERT for block group's memory leak") introduced these
assertions.

So fix this by freeing the block groups only after waiting for all
worker kthreads to complete and shutdown the workqueues.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>