smb3: fix problem remounting a share after shutdown

xfstests generic/392 showed a problem where even after a
shutdown call was made on a mount, we would still attempt
to use the (now inaccessible) superblock if another mount
was attempted for the same share.

Reported-by: David Howells <dhowells@redhat.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Cc: <stable@vger.kernel.org>
Fixes: 087f757b01 ("cifs: add shutdown support")
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/connect.c

@@ -2709,6 +2709,13 @@ cifs_match_super(struct super_block *sb, void *data)
 
 	spin_lock(&cifs_tcp_ses_lock);
 	cifs_sb = CIFS_SB(sb);
+
+	/* We do not want to use a superblock that has been shutdown */
+	if (CIFS_MOUNT_SHUTDOWN & cifs_sb->mnt_cifs_flags) {
+		spin_unlock(&cifs_tcp_ses_lock);
+		return 0;
+	}
+
 	tlink = cifs_get_tlink(cifs_sb_master_tlink(cifs_sb));
 	if (tlink == NULL) {
 		/* can not match superblock if tlink were ever null */