if_vxlan(4): Exclude ETHER_CRC_LEN from macro VXLAN_MAX_MTU

The encapsulated (original) frame does not count in FCS as per Section 5
of RFC 7348.

Reviewed by: afedorov, bryanv, network
Fixes: b7592822d5de Allow set MTU more than 1500 bytes
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45195