f2fs: fix to avoid REQ_TIME and CP_TIME collision

f2fs: fix to avoid REQ_TIME and CP_TIME collision

[ Upstream commit 493720a4854343b7c3fe100cda6a3a2c3f8d4b5d ]

Lei Li reported a issue: if foreground operations are frequent, background
checkpoint may be always skipped due to below check, result in losing more
data after sudden power-cut.

f2fs_balance_fs_bg()
...
	if (!is_idle(sbi, REQ_TIME) &&
		(!excess_dirty_nats(sbi) && !excess_dirty_nodes(sbi)))
		return;

E.g:
cp_interval = 5 second
idle_interval = 2 second
foreground operation interval = 1 second (append 1 byte per second into file)

In such case, no matter when it calls f2fs_balance_fs_bg(), is_idle(, REQ_TIME)
returns false, result in skipping background checkpoint.

This patch changes as below to make trigger condition being more reasonable:
- trigger sync_fs() if dirty_{nats,nodes} and prefree segs exceeds threshold;
- skip triggering sync_fs() if there is any background inflight IO or there is
foreground operation recently and meanwhile cp_rwsem is being held by someone;

Reported-by: Lei Li <noctis.akm@gmail.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Stable-dep-of: d80afefb17e0 ("f2fs: fix to account FS_CP_DATA_IO correctly")
Signed-off-by: Sasha Levin <sashal@kernel.org>