conntrack: Fix ct-clean thread crash bug.

conntrack: Fix ct-clean thread crash bug.

Conn should be removed from the connection expiry list when
the connection tracker experiences NAT resource exhaustion
and the connection needing NAT mapping cannot get it.
If this is not done, the connection tracker can crash during
cleanup of expired connections by the clean thread.

This crash will be triggered when a established flow do ct(nat)
again, like
"ip,actions=ct(table=1)
 table=1,in_port=1,ip,actions=ct(commit,nat(dst=5.5.5.5)),2
 table=1,in_port=2,ip,ct_state=+est,actions=1
 table=1,in_port=1,ip,ct_state=+est,actions=2"

Fixes: bd5e81a0e596 ("Userspace Datapath: Add ALG infra and FTP.")
Signed-off-by: Lili Huang <huanglili.huang@huawei.com>
Signed-off-by: Darrell Ball <dlu998@gmail.com>