if_vxlan(4): Prefer SYSCTL_INT over TUNABLE_INT

This driver does not need to retrieve those tunable during early boot.
Meanwhile SYSCTL_INT can provide rich info such as description.
Also sysctl net.link.vxlan.[legacy_port|reuse_port] can report the
current settings.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48621

(cherry picked from commit 92632371b360010709fad60146f1aee0b8b99776)