vfs: show_vfsstat: do not ignore errors from show_devname method

commit 5f8d498d upstream.

Explicitly check show_devname method return code and bail out in case
of an error.  This fixes regression introduced by commit 9d4d6574.
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[bwh: Backported to 3.16: add the 'out' label]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/proc_namespace.c

@@ -191,6 +191,8 @@ static int show_vfsstat(struct seq_file *m, struct vfsmount *mnt)
 	if (sb->s_op->show_devname) {
 		seq_puts(m, "device ");
 		err = sb->s_op->show_devname(m, mnt_path.dentry);
+		if (err)
+			goto out;
 	} else {
 		if (r->mnt_devname) {
 			seq_puts(m, "device ");
@@ -216,6 +218,7 @@ static int show_vfsstat(struct seq_file *m, struct vfsmount *mnt)
 	}
 
 	seq_putc(m, '\n');
+out:
 	return err;
 }