SUNRPC: introduce svc_xprt_create_from_sa utility routine

Add svc_xprt_create_from_sa utility routine and refactor
svc_xprt_create() codebase in order to introduce the capability to
create a svc port from socket address.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>