The "NextProtocol" field was added as an unconditional field which made the
VXLAN packet header consume an additional 8-bts when used in a Legacy VXLAN
deployments (flags=0x08) or in GPID deployments (flags=0x88).  The resulting
regression caused incorrect parsing of subsequent headers.  For example, if an
Ethernet header followed the VXLAN header then it would not be interpreted as
an Ethernet header.  The actually interpretation would depend on what value was
stored in the Ether.dst field.

This change updates the field layout specification such that the NextProtocol
field is conditional on flags=0x04, and shortens the reserved field by 8-bits
if that condition is true.

fixes bug #281

Signed-off-by: Allain Legacy <allain.legacy@windriver.com>