f2fs: add a punch discard command function

This patch add a function to punch discard command if one segment
reuse before discard. Split this segment from multi-segments discard
range, and discard the left bigger range.

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>