SUNRPC: rpc_create() default hostname should support AF_INET6 addresses

If the ULP doesn't pass a hostname string to rpc_create(), it manufactures
one based on the passed-in address.  Be smart enough to handle an AF_INET6
address properly in this case.

Move the default servername logic before the xprt_create_transport() call
to simplify error handling in rpc_create().

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