interface: Add missing ip4 udp->checksum = 0 prior to computing checksum
interface: Add missing ip4 udp->checksum = 0 prior to computing checksum

For ip4 tcp, ip6 tcp, and ip6 udp packet, we set checksum = 0 prior to
computing the checksum. We missed ip4 udp case. This oversight requires all
clients to set udp->checksum = 0 if ip4 udp checksum offload is needed.

Type: fix

Signed-off-by: Steven Luong <a748ea44ad7b179c2313a856d44e1e4eb39869bb@cisco.com>
Change-Id: Ic608811e82099f3bec469e123671e9b281f38d76

chrisy/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,chrisy/vpp