f2fs: prepare a waiter before entering io_schedule

This is to avoid sleep() in the waiter thread.

[   20.157753] ------------[ cut here ]------------
[   20.158393] do not call blocking ops when !TASK_RUNNING; state=2 set at [<0000000096354225>] prepare_to_wait+0xcd/0x430
[   20.159858] WARNING: CPU: 1 PID: 1152 at kernel/sched/core.c:7142 __might_sleep+0x149/0x1a0
...
[   20.176110]  __submit_merged_write_cond+0x191/0x310
[   20.176739]  f2fs_submit_merged_write+0x18/0x20
[   20.177323]  f2fs_wait_on_all_pages+0x269/0x2d0
[   20.177899]  ? block_operations+0x980/0x980
[   20.178441]  ? __kasan_check_read+0x11/0x20
[   20.178975]  ? finish_wait+0x260/0x260
[   20.179488]  ? percpu_counter_set+0x147/0x230
[   20.180049]  do_checkpoint+0x1757/0x2a50
[   20.180558]  f2fs_write_checkpoint+0x840/0xaf0
[   20.181126]  f2fs_sync_fs+0x287/0x4a0.

Change-Id: I8ec83957172abca950945cafa6c4311d8ec08a59
Reported-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Git-commit: 828add774f0d2bf930cdeca6c982c1fbcdd846bb
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/


Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>