UBI: fix soft lockup in ubi_check_volume()

UBI: fix soft lockup in ubi_check_volume()

Running mtd-utils/tests/ubi-tests/io_basic.c could cause
soft lockup or watchdog reset. It is because *updatevol*
will perform ubi_check_volume() after updating finish
and this function will full scan the updated lebs if the
volume is initialized as STATIC_VOLUME.

This patch adds *cond_resched()* in the loop of lebs scan
to avoid soft lockup.

Helped by Richard Weinberger <richard@nod.at>

[ 2158.067096] INFO: rcu_sched self-detected stall on CPU { 1}  (t=2101 jiffies g=1606 c=1605 q=56)
[ 2158.172867] CPU: 1 PID: 2073 Comm: io_basic Tainted: G           O 3.10.53 #21
[ 2158.172898] [<c000f624>] (unwind_backtrace+0x0/0x120) from [<c000c294>] (show_stack+0x10/0x14)
[ 2158.172918] [<c000c294>] (show_stack+0x10/0x14) from [<c008ac3c>] (rcu_check_callbacks+0x1c0/0x660)
[ 2158.172936] [<c008ac3c>] (rcu_check_callbacks+0x1c0/0x660) from [<c002b480>] (update_process_times+0x38/0x64)
[ 2158.172953] [<c002b480>] (update_process_times+0x38/0x64) from [<c005ff38>] (tick_sched_handle+0x54/0x60)
[ 2158.172966] [<c005ff38>] (tick_sched_handle+0x54/0x60) from [<c00601ac>] (tick_sched_timer+0x44/0x74)
[ 2158.172978] [<c00601ac>] (tick_sched_timer+0x44/0x74) from [<c003f348>] (__run_hrtimer+0xc8/0x1b8)
[ 2158.172992] [<c003f348>] (__run_hrtimer+0xc8/0x1b8) from [<c003fd9c>] (hrtimer_interrupt+0x128/0x2a4)
[ 2158.173007] [<c003fd9c>] (hrtimer_interrupt+0x128/0x2a4) from [<c0246f1c>] (arch_timer_handler_virt+0x28/0x30)
[ 2158.173022] [<c0246f1c>] (arch_timer_handler_virt+0x28/0x30) from [<c0086214>] (handle_percpu_devid_irq+0x9c/0x124)
[ 2158.173036] [<c0086214>] (handle_percpu_devid_irq+0x9c/0x124) from [<c0082bd8>] (generic_handle_irq+0x20/0x30)
[ 2158.173049] [<c0082bd8>] (generic_handle_irq+0x20/0x30) from [<c000969c>] (handle_IRQ+0x64/0x8c)
[ 2158.173060] [<c000969c>] (handle_IRQ+0x64/0x8c) from [<c0008544>] (gic_handle_irq+0x3c/0x60)
[ 2158.173074] [<c0008544>] (gic_handle_irq+0x3c/0x60) from [<c02f0f80>] (__irq_svc+0x40/0x50)
[ 2158.173083] Exception stack(0xc4043c98 to 0xc4043ce0)
[ 2158.173092] 3c80:                                                       c4043ce4 00000019
[ 2158.173102] 3ca0: 1f8a865f c050ad10 1f8a864c 00000031 c04b5970 0003ebce 00000000 f3550000
[ 2158.173113] 3cc0: bf00bc68 00000800 0003ebce c4043ce0 c0186d14 c0186cb8 80000013 ffffffff
[ 2158.173130] [<c02f0f80>] (__irq_svc+0x40/0x50) from [<c0186cb8>] (read_current_timer+0x4/0x38)
[ 2158.173145] [<c0186cb8>] (read_current_timer+0x4/0x38) from [<1f8a865f>] (0x1f8a865f)
[ 2183.927097] BUG: soft lockup - CPU#1 stuck for 22s! [io_basic:2073]
[ 2184.002229] Modules linked in: nandflash(O) [last unloaded: nandflash]

Signed-off-by: Wang Kai <morgan.wang@huawei.com>
Signed-off-by: hujianyang <hujianyang@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>