MFC: 1.72: Prevent cached rtentry from being removed, add rtentry locking to nd6_output().
MFC: 1.72: Prevent cached rtentry from being removed, add rtentry
locking to nd6_output().

PR:		kern/93910, kern/105437

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