f2fs: issue small discard by LBA order

For small granularity discard which size is smaller than 64KB, if we
issue those kind of discards orderly by size, their IOs will be spread
into entire logical address, so that in FTL, L2P table will be updated
randomly, result bad wear rate in the table.

In this patch, we choose to issue small discard by LBA order, by this
way, we can expect that L2P table updates from adjacent discard IOs can
be merged in the cache, so it can reduce lifetime wearing of flash.

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