[NETFILTER]: tcp conntrack: accept SYN|URG as valid

Some stacks apparently send packets with SYN|URG set. Linux accepts
these packets, so TCP conntrack should to.

Pointed out by Martijn Posthuma <posthuma@sangine.com>.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>