amt: fix possible memory leak in amt_rcv()

If an amt receives packets and it finds socket.
If it can't find a socket, it should free a received skb.
But it doesn't.
So, a memory leak would possibly 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>