sunrpc: Don't return NULL from rpcb_create

> The reason for this is in the future, we may want to support additional
> address family types.  We should, therefore, ensure that every piece of
> code that is sensitive to address families fail in some orderly manner
> to let developers know where a change is needed.

Makes sense. I was under impression, that AF-s other than INET are not
cared about at all :(

Here's a fixed version of the patch.

Log:

Its callers check for ERR_PTR.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>