[NETFILTER]: TCP conntrack: accept RST|PSH as valid

This combination has been encountered on an IBM AS/400 in response
to packets sent to a closed session. There is no particular reason
to mark it invalid.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>