init: Calculate correct maximum mbuf size

Maximum mbuf size must be the maximum packet size plus
any prepended data that is added when passing to and from
vswitchd.

Change-Id: I05e59c87de1ad9f32695086f0bdab3a5fd67193e
Reviewed-by: Nicolas Pernas Maradei <nicolasx.pernas.maradei@intel.com>
Signed-off-by: Mark D. Gray <mark.d.gray@intel.com>