cifs: Allow LANMAN auth method for servers supporting unencapsulated authentication methods

This allows users to use LANMAN authentication on servers which support
unencapsulated authentication.

The patch fixes a regression where users using plaintext authentication
were no longer able to do so because of changed bought in by patch
3f618223dc0bdcbc8d510350e78ee2195ff93768

https://bugzilla.redhat.com/show_bug.cgi?id=1011621

Reported-by: Panos Kavalagios <[e-mail]>
Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Sachin Prabhu <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/sess.c

···
 				return NTLMv2;
 			if (global_secflags & CIFSSEC_MAY_NTLM)
 				return NTLM;
-			/* Fallthrough */
 		default:
-			return Unspecified;
+			/* Fallthrough to attempt LANMAN authentication next */
+			break;
 		}
 	case CIFS_NEGFLAVOR_LANMAN:
 		switch (requested) {