f2fs: change segment to section in f2fs_ioc_gc_range

f2fs_ioc_gc_range skips blocks_per_seg each time, however, f2fs_gc moves
blocks of section each time, so fix it from segment to section.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>