nhrpd: fixes duplicate auth extension

nhrpd: fixes duplicate auth extension

When an NHRP peer was forwarding a message, it was copying all
extensions from the originally received packet. The authentication
extension must be regenerated hop by hop per RFC2332.
This fix checks for the auth extension when copying extensions
and omits the original packet auth and instead regenerates a new auth extension.

Fix bug #16507

Signed-off-by: Denys Haryachyy <garyachy@gmail.com>
(cherry picked from commit 8e3c278bbcd0ced1d4058cc7a2c9aebdfbc8b651)