f2fs: stop issuing discard immediately if there is queued IO

For background discard policy, even if there is queued user IO, still
we will check max_requests times for next discard entry, it is unneeded,
let's just stop this round submission immediately.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>