Merge r1.22-1.23 from NetBSD:

Merge r1.22-1.23 from NetBSD:
  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