f2fs: remove duplicated code in sanity_check_area_boundary

Use seg_end_blkaddr instead of "segment0_blkaddr + (segment_count <<
log_blocks_per_seg)".

Signed-off-by: Wang Xiaojun <wangxiaojun11@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>