Close some but not all writer-writer races when maintaining IPv6 interface address lists by locking the interface address list lock.
Close some but not all writer-writer races when maintaining IPv6
interface address lists by locking the interface address list lock.

MFC after:	2 weeks

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