MFC r340260 (emaste):

MFC r340260 (emaste):
 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.

Approved by:	re (kib, insta-MFC)
Security:	CVE-2018-17156
Sponsored by:	The FreeBSD Foundation