net/ixgbe: add compile-time checks to vector driver

net/ixgbe: add compile-time checks to vector driver

The vector PMD is very sensitive to the layout of the mbuf, especially on
the RX path. Add in some compile-time checks to make sure the mbuf layout
assumptions are valid, and to provide hints to anyone changing the mbuf
where things may need to be updated.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>