if_vxlan(4): Check the size of data available in mbuf before using them

PR: 261711
Reviewed by: kp
Tested by: Alfa (burak.sn at outlook.com)
Approved by: re (gjb), kp (mentor)
Differential Revision: https://reviews.freebsd.org/D36794

(cherry picked from commit 8707cb19e68128c181477d87f960182c14e15f12)
(cherry picked from commit 639aebb2aa0b72c569f8779e13351d37f7bb17c6)