f2fs: code cleanup for f2fs_statfs_project()

Calling min_not_zero() to simplify complicated prjquota
limit comparison in f2fs_statfs_project().

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>