smb3: on kerberos mount if server doesn't specify auth type use krb5

commit 926674de6705f0f1dbf29a62fd758d0977f535d6 upstream.

Some servers (e.g. Azure) do not include a spnego blob in the SMB3
negotiate protocol response, so on kerberos mounts ("sec=krb5")
we can fail, as we expected the server to list its supported
auth types (OIDs in the spnego blob in the negprot response).
Change this so that on krb5 mounts we default to trying krb5 if the
server doesn't list its supported protocol mechanisms.
Signed-off-by: NSteve French <stfrench@microsoft.com>
Reviewed-by: NRonnie Sahlberg <lsahlber@redhat.com>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/cifs/cifs_spnego.c

@@ -147,8 +147,10 @@ cifs_get_spnego_key(struct cifs_ses *sesInfo)
 		sprintf(dp, ";sec=krb5");
 	else if (server->sec_mskerberos)
 		sprintf(dp, ";sec=mskrb5");
-	else
-		goto out;
+	else {
+		cifs_dbg(VFS, "unknown or missing server auth type, use krb5\n");
+		sprintf(dp, ";sec=krb5");
+	}
 
 	dp = description + strlen(description);
 	sprintf(dp, ";uid=0x%x",