vfs: show_vfsstat: do not ignore errors from show_devname method

Explicitly check show_devname method return code and bail out in case
of an error.  This fixes regression introduced by commit 9d4d65748a5c.

Cc: stable@vger.kernel.org
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c
index 2256e7e..3f1190d 100644
--- a/fs/proc_namespace.c
+++ b/fs/proc_namespace.c
@@ -199,6 +199,8 @@
 	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 ");