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
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45197