if_vxlan(4): Correct the statistic for output bytes

The vxlan interface encapsulates the Ethernet frame by prepending IP/UDP
and vxlan headers. For statistics, only the payload, i.e. the
encapsulated (inner) frame should be counted.

Event: Aberdeen Hackathon 2022
Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D36855