f2fs: avoid duplicated codes for cleanup

f2fs_segment_has_free_slot() was copied and modified from
__next_free_blkoff(), they are almost the same, clean up to
reuse common code as much as possible.

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