sunrpc: Factor out udp sockets creation

Make it look like the TCP sockets creation.
Unfortunately the git diff made the patch look messy :(

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>