amt: fix memory leak for advertisement message

When a gateway receives an advertisement message, it extracts relay
information and then it should be freed.
But the advertisement handler doesn't free it.
So, memory leak would occur.

Fixes: cbc21dc1 ("amt: add data plane of amt interface")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>