SUNRPC: Don't recognize RPC_AUTH_MAXFLAVOR

RPC_AUTH_MAXFLAVOR is an invalid flavor, on purpose.  Don't allow
any processing whatsoever if a caller passes it to rpcauth_create()
or rpcauth_get_gssinfo().

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>