rxrpc: Fix loss of final ack on shutdown

Fix the loss of transmission of a call's final ack when a socket gets shut
down.  This means that the server will retransmit the last data packet or
send a ping ack and then get an ICMP indicating the port got closed.  The
server will then view this as a failure.

Fixes: 3136ef49

 ("rxrpc: Delay terminal ACK transmission on a client call")
Signed-off-by: David Howells <dhowells@redhat.com>