amt: fix return value of amt_update_handler()

If a relay receives an update message, it lookup a tunnel.
and if there is no tunnel for that message, it should be treated
as an error, not a success.
But amt_update_handler() returns false, which means success.

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>