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
Approved by: kp (mentor)
Differential Revision: https://reviews.freebsd.org/D36855

(cherry picked from commit de1ea2d5171ec53acd69f93821b9507267ee44b7)