if_ovpn: ensure it's safe to modify the mbuf

PR: 280036
Reviewed by: ae
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46529

(cherry picked from commit 5644e2c6d47c6113a61ab7fc0776b7227677656a)