f2fs: code cleanup for f2fs_statfs_project()

commit bf2cbd3c57159c2b639ee8797b52ab5af180bf83 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>