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)

This is wrong: m_adj() will modify m1->m_len, so we're using a wrong
value when manually adjusting m->m_pkthdr.len.