vxlan: move inner L2 header processing to a separate function

This code will be different for VXLAN-GPE, so move it to a separate
function. It will also make the rx path less spaghetti-like.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>