header parser uses m_pullup(9) to get access to mbuf chain.
m_pullup(9) can allocate new mbuf chain and free old one if the
space left in the mbuf chain is not enough to hold requested
contiguous bytes. Previously drivers can use stale ip/tcp header
pointer if m_pullup(9) returned new mbuf chain.
Reported by: Andrew Boyer (aboyer <> averesystems dot com)
MFC after: 10 days