vxlan: Add support for socket ioctls SIOC[SG]TUNFIB

Submitted by: Luiz Amaral <email@luiz.eng.br>
PR: 244004
Reviewed by: gbe (manpages), melifaro, pauamma (manpages)
Approved by: kp (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D32820

(cherry picked from commit 7f7a804ae077a0c43d8c3f1444f90bdfb841c6b1)