fuse: always allow query of st_dev

Fuse mounts without "allow_other" are off-limits to all non-owners.  Yet it
makes sense to allow querying st_dev on the root, since this value is
provided by the kernel, not the userspace filesystem.

Allow statx(2) with a zero request mask to succeed on a fuse mounts for all
users.
Reported-by: Nikolaus Rath <Nikolaus@rath.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dir.c

@@ -1689,8 +1689,18 @@ static int fuse_getattr(const struct path *path, struct kstat *stat,
 	struct inode *inode = d_inode(path->dentry);
 	struct fuse_conn *fc = get_fuse_conn(inode);
 
-	if (!fuse_allow_current_process(fc))
+	if (!fuse_allow_current_process(fc)) {
+		if (!request_mask) {
+			/*
+			 * If user explicitly requested *nothing* then don't
+			 * error out, but return st_dev only.
+			 */
+			stat->result_mask = 0;
+			stat->dev = inode->i_sb->s_dev;
+			return 0;
+		}
 		return -EACCES;
+	}
 
 	return fuse_update_get_attr(inode, NULL, stat, request_mask, flags);
 }