MFC r279393:

MFC r279393:

Pad RX copy alignment calculation to avoid illegal memory accesses

The optimization made in r239940 is valid for struct mbuf's current structure
and size in FreeBSD, but hardcodes assumptions about sizes of struct mbuf,
which are unfortunately broken if additional data is added to the beginning of
struct mbuf

X-MFC note (discussed with rwatson):

This change requires the MPKTHSIZE definition, which is only available after
head@r277203 and will not be MFCed as it breaks mbuf(9) KPI.

A direct commit to stable/10 and merges to other branches to add the necessary
definitions to work with the code as-is will be done to facilitate this MFC

PR: 194314
Approved/Reviewed by: erj, jfv
Sponsored by: EMC / Isilon Storage Division

git-svn-id: svn://svn.freebsd.org/base/stable/10@281954 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f