[NETFILTER]: H.323 helper: replace internal_net_addr parameter by routing-based heuristic

[NETFILTER]: H.323 helper: replace internal_net_addr parameter by routing-based heuristic

Call Forwarding doesn't need to create an expectation if both peers can
reach each other without our help. The internal_net_addr parameter
lets the user explicitly specify a single network where this is true,
but is not very flexible and even fails in the common case that calls
will both be forwarded to outside parties and inside parties. Use an
optional heuristic based on routing instead, the assumption is that
if bpth the outgoing device and the gateway are equal, both peers can
reach each other directly.

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