Add context pointer and source address to the UDP tunnel callback

These are needed for the forthcoming vxlan implementation. The context
pointer means we do not have to use a spare pointer field in the inpcb,
and the source address is required to populate vxlan's forwarding table.

While I highly doubt there is an out of tree consumer of the UDP
tunneling callback, this change may be a difficult to eventually MFC.

Phabricator:	https://reviews.freebsd.org/D383
Reviewed by:	gnn
Make sure SIZE is defined early