fib_trie: Fix warning on fib4_rules_exit

fib_trie: Fix warning on fib4_rules_exit

This fixes the following warning:

 BUG: sleeping function called from invalid context at mm/slub.c:1268
 in_atomic(): 1, irqs_disabled(): 0, pid: 6, name: kworker/u8:0
 INFO: lockdep is turned off.
 CPU: 3 PID: 6 Comm: kworker/u8:0 Tainted: G        W       4.0.0-rc5+ #895
 Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
 Workqueue: netns cleanup_net
  0000000000000006 ffff88011953fa68 ffffffff81a203b6 000000002c3a2c39
  ffff88011952a680 ffff88011953fa98 ffffffff8109daf0 ffff8801186c6aa8
  ffffffff81fbc9e5 00000000000004f4 0000000000000000 ffff88011953fac8
 Call Trace:
  [<ffffffff81a203b6>] dump_stack+0x4c/0x65
  [<ffffffff8109daf0>] ___might_sleep+0x1c3/0x1cb
  [<ffffffff8109db70>] __might_sleep+0x78/0x80
  [<ffffffff8117a60e>] slab_pre_alloc_hook+0x31/0x8f
  [<ffffffff8117d4f6>] __kmalloc+0x69/0x14e
  [<ffffffff818ed0e1>] ? kzalloc.constprop.20+0xe/0x10
  [<ffffffff818ed0e1>] kzalloc.constprop.20+0xe/0x10
  [<ffffffff818ef622>] fib_trie_table+0x27/0x8b
  [<ffffffff818ef6bd>] fib_trie_unmerge+0x37/0x2a6
  [<ffffffff810b06e1>] ? arch_local_irq_save+0x9/0xc
  [<ffffffff818e9793>] fib_unmerge+0x2d/0xb3
  [<ffffffff818f5f56>] fib4_rule_delete+0x1f/0x52
  [<ffffffff817f1c3f>] ? fib_rules_unregister+0x30/0xb2
  [<ffffffff817f1c8b>] fib_rules_unregister+0x7c/0xb2
  [<ffffffff818f64a1>] fib4_rules_exit+0x15/0x18
  [<ffffffff818e8c0a>] ip_fib_net_exit+0x23/0xf2
  [<ffffffff818e91f8>] fib_net_exit+0x32/0x36
  [<ffffffff817c8352>] ops_exit_list+0x45/0x57
  [<ffffffff817c8d3d>] cleanup_net+0x13c/0x1cd
  [<ffffffff8108b05d>] process_one_work+0x255/0x4ad
  [<ffffffff8108af69>] ? process_one_work+0x161/0x4ad
  [<ffffffff8108b4b1>] worker_thread+0x1cd/0x2ab
  [<ffffffff8108b2e4>] ? process_scheduled_works+0x2f/0x2f
  [<ffffffff81090686>] kthread+0xd4/0xdc
  [<ffffffff8109ec8f>] ? local_clock+0x19/0x22
  [<ffffffff810905b2>] ? __kthread_parkme+0x83/0x83
  [<ffffffff81a2c0c8>] ret_from_fork+0x58/0x90
  [<ffffffff810905b2>] ? __kthread_parkme+0x83/0x83

The issue was that as a part of exiting the default rules were being
deleted which resulted in the local trie being unmerged.  By moving the
freeing of the FIB tables up we can avoid the unmerge since there is no
local table left when we call the fib4_rules_exit function.

Fixes: 0ddcf43d5d4a ("ipv4: FIB Local/MAIN table collapse")
Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>