sunrpc: Merge the xs_bind code

There's the only difference betseen the xs_bind4 and the
xs_bind6 - the size of sockaddr structure they use.

Fortunatelly its size can be indirectly get from the transport.

Change since v1:
* use sockaddr_storage instead of sockaddr
* use rpc_set_port instead of manual port assigning

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