conntrack: Tighten handling of alg reverse conns.

conntrack: Tighten handling of alg reverse conns.

Close a theoretical race delete/create corner case for alg
reverse conns and add debugging around this that may point to
an intentional exploit, unintentional problem or just a rare
condition. The solution is to keep track of reverse conn via
nat_conn_keys and avoid deleting the reverse conn when it has been
recreated.

Fixes: bd5e81a0e596 ("Userspace Datapath: Add ALG infra and FTP.")
Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>