erofs: derive fsid from on-disk UUID for .statfs() if possible

Use the superblock's UUID to generate the fsid when it's non-null.
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Hongzhen Luo <hongzhen@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240409113022.74720-1-hongzhen@linux.alibaba.comSigned-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/super.c

@@ -914,22 +914,20 @@ static int erofs_statfs(struct dentry *dentry, struct kstatfs *buf)
 {
 	struct super_block *sb = dentry->d_sb;
 	struct erofs_sb_info *sbi = EROFS_SB(sb);
-	u64 id = 0;
-
-	if (!erofs_is_fscache_mode(sb))
-		id = huge_encode_dev(sb->s_bdev->bd_dev);
 
 	buf->f_type = sb->s_magic;
 	buf->f_bsize = sb->s_blocksize;
 	buf->f_blocks = sbi->total_blocks;
 	buf->f_bfree = buf->f_bavail = 0;
-
 	buf->f_files = ULLONG_MAX;
 	buf->f_ffree = ULLONG_MAX - sbi->inos;
-
 	buf->f_namelen = EROFS_NAME_LEN;
 
-	buf->f_fsid    = u64_to_fsid(id);
+	if (uuid_is_null(&sb->s_uuid))
+		buf->f_fsid = u64_to_fsid(erofs_is_fscache_mode(sb) ? 0 :
+				huge_encode_dev(sb->s_bdev->bd_dev));
+	else
+		buf->f_fsid = uuid_to_fsid(sb->s_uuid.b);
 	return 0;
 }