[NETFILTER]: SIP helper: expect RTP streams in both directions

Since we don't know in which direction the first packet will arrive, we
need to create one expectation for each direction, which is currently
prevented by max_expected beeing set to 1.

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