SUNRPC: Add a convenient default for the hostname when calling rpc_create()

A couple of callers just use a stringified IP address for the rpc client's
hostname.  Move the logic for constructing this into rpc_create(), so it can
be shared.

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