Pad RX copy alignment calculation to avoid illegal memory accesses when copying custom mbuf structures

[ngie]

Summary

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

PR: 194314
MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division

Test Plan

Preconditions:

• System must have an ixgbe interface
• System must be on a network with a DHCP server

• Adjust struct mbuf (add dummy int * values in the header field).
• Rebuild the kernel and build ixgbe as a module, not statically compiled into the kernel
• Boot system with custom kernel into singleuser mode
• Press <Enter>
• kldload ixgbe
• dhclient ix0

Expected results:

• The interface should be brought up
• An IP address should be assigned to the interface

Actual results:
The dhclient ix0 call would deterministically panic the system with page faults
in the ixgbe code

[jfv]

Looks good.

[erj]

I'd like an #ifndef MPKTHSIZE block to define what MPKTHSIZE is for pre-11 FreeBSD versions, just in case someone takes this driver to use on 10.1; but as it is I think it's good. I tried the sample application you posted in the PR on 10.1 amd64, 11 amd64, and 11 i386 and I didn't get any asserts.

[ngie]

Fix committed in r279393 -- thank you!