Fix assertion for write netlock in rip6_input(). ip6_input() has

Fix assertion for write netlock in rip6_input().  ip6_input() has
shared net lock.  ip_deliver() needs exclusive net lock.  Instead
of calling ip_deliver() directly, use ip6_ours() to queue the packet.
Move the write lock assertion into ip_deliver() to catch such bugs
earlier.
The assertion was only triggered with IPv6 multicast forwarding or
router alert hop by hop option.  Found by regress test.
OK kn@ mvs@