f2fs: change return value of reserved_segments to unsigned int

The type of SM_I(sbi)->reserved_segments is unsigned int,
so change the return value to unsigned int.
The type cast can be removed in reserved_sections as a result.

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