Avoid buffer underwrite in icmp_error

Avoid buffer underwrite in icmp_error

icmp_error allocates either an mbuf (with pkthdr) or a cluster depending
on the size of data to be quoted in the ICMP reply, but the calculation
failed to account for the additional padding that m_align may apply.

Include the ip header in the size passed to m_align.  On 64-bit archs
this will have the net effect of moving everything 4 bytes later in the
mbuf or cluster.  This will result in slightly pessimal alignment for
the ICMP data copy.

Also add an assertion that we do not move m_data before the beginning of
the mbuf or cluster.

Reported by:	A reddit user
Reviewed by:	bz, jtl
MFC after:	3 days
Security:	CVE-2018-17156
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D17909