incorrectly output, if the RB-tree enumeration happened to reuse the
same chain for a mode switch: that is, both ALLOW and BLOCK records
were appended for the same group, in the same mbuf packet chain.
This was introduced during an mbuf chain layout bug fix involving
m_getptr(), which obviously cannot count from offset 0 on the
second pass through the RB-tree when serializing the IGMPv3
group records into the pending mbuf chain.
Cut over to KTR_INET for IGMPv3 CTR usage.
31a3e65dc2