correct PCB locking in broadcast/multicast case that was exposed by change to use udp_append
correct PCB locking in broadcast/multicast case that was exposed by change
to use udp_append

Reviewed by:	hsu

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase