Geneve: Modernize packet parsing style.

Geneve: Modernize packet parsing style.

Enable ND_LONGJMP_FROM_TCHECK. Remove one redundant ND_TCHECK_*()
instance and make another one conditional. Report invalid packets as
invalid. Test that a non-Ethernet payload is fully within the packet
buffer. Add a length check to geneve_opts_print() and have it indicate
invalid tunnel options. Constify an argument. Update the packet diagrams
and refer to the RFC rather than the draft.