if_vxlan(4): Allow netmap_generic to intercept RX packets.

Netmap (generic) intercepts the if_input method to handle RX packets.

Call ifp->if_input() instead of netisr_dispatch().
Add stricter check for incoming packet length.

This change is very useful with bhyve + vale + if_vxlan.

Reviewed by: vmaffione (mentor), kib, np, donner
Approved by: vmaffione (mentor), kib, np, donner
MFC after: 2 weeks
Sponsored by: vstack.com
Differential Revision: https://reviews.freebsd.org/D30638