Pull up following revision(s) (requested by maxv in ticket #773):

sys/netipsec/ipsec_mbuf.c: revision 1.23,1.24

Don't assume M_PKTHDR is set only on the first mbuf of the chain. It
should, but it looks like there are several places that can put M_PKTHDR
on secondary mbufs (PR/53189), so drop this assumption right now to
prevent further bugs.

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.

Because of that, it is possible to exploit the attack I described in
uipc_mbuf.c::rev1.182. The exploit is more complicated, but works 100%
reliably.

(martin)