if_vxlan(4): Add checking for loops and nesting of tunnels

User misconfiguration, either tunnel loops, or a large number of
different nested tunnels, can overflow the kernel stack. Prevent that
by using if_tunnel_check_nesting().

PR: 278394
Diagnosed by: markj
Reviewed by: kp
Approved by: re (cperciva)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45197

(cherry picked from commit 93fbfef0b50354b7a1620822454ef29cd415cb2d)
(cherry picked from commit 3ebd2b1c730834123a53b3eddcf9029fcf414782)