Don't assume M_PKTHDR is set only on the first mbuf of the chain.
The check is replaced by (m1 != m), which is equivalent to the previous
code: we want to modify m->m_pkthdr.len only when 'm' was not passed in
m_adj().
Fix a pretty bad mistake, that has always been there:
m_adj(m1, -(m1->m_len - roff));
if (m1 != m)
m->m_pkthdr.len -= (m1->m_len - roff);
This is wrong: m_adj() will modify m1->m_len, so we're using a wrong
value when manually adjusting m->m_pkthdr.len.
Reported by: Maxime Villard <max at m00nbsd dot net>
Obtained from: NetBSD
MFC after: 1 week
bf18dfa28e