if_vxlan(4): Prefer SYSCTL_INT over TUNABLE_INT

[zlei]

Summary

This driver does not need to retrieve those tunable during early boot. Meanwhile SYSCTL_INT can provide rich meta info such as description. sysctl net.link.vxlan will also report current setting.

MFC after: 1 week

Test Plan

# kldload if_vxlan
# sysctl net.link.vxlan
net.link.vxlan.legacy_port: 0
net.link.vxlan.reuse_port: 0