tipc: split UDP send function

Split the UDP send function into two. One callback that prepares the
skb and one transmit function that sends the skb. This will come in
handy in later patches, when we introduce UDP replicast.

Signed-off-by: Richard Alpe <richard.alpe@ericsson.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>