Rather than checking for hlen causing misalignment, we should do the
m_adj() and then check the resulting mbuf for misalignment, copying
backwards to align the mbuf if required.

This fixes a longstanding problem where an mbuf which would have been
properly aligned after an m_adj() was being misaligned and causing an
unaligned access trap in ip_input(). This bug only triggered when booting
diskless.

Reviewed by: dfr