MFC r290441:

MFC r290441:
Fix for unaligned IP-header.

The mbuf length fields must be set before m_adj() is called else
m_adj() will not always adjust the mbuf and an unaligned read
exception can trigger inside the network stack. This can happen on
platforms where unaligned reads are not supported. Adjust a length
check to include the 2-byte ethernet alignment while at it.

git-svn-id: svn://svn.freebsd.org/base/stable/8@290609 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f