f2fs: schedule in between two continous batch discards

In batch discard approach of fstrim will grab/release gc_mutex lock
repeatly, it makes contention of the lock becoming more intensive.

So after one batch discards were issued in checkpoint and the lock
was released, it's better to do schedule() to increase opportunity
of grabbing gc_mutex lock for other competitors.

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