Fix IP spoofing filter blocking all packets.

Implement IP spoofing filter by adding yet another chain to iptables
with RETURN rule for each of IP addresses assigned to port and DROP
rule at the end of the chain.

Fixes: bug #1190613
Change-Id: I2e7a0f8dd4c3519c57d28e87a44735a3482624b1
Make sure exceptions during policy checks are logged.

If the invocation of f bombs out, the policy check fails (i.e. returns
False), however it does not log the root cause, which makes very
difficult to understand why this is happening.

Fixes bug #1191948

Change-Id: Ic40053f3965b71199baf9fe3902e8ffc9745076f
Merge "Fix IP spoofing filter blocking all packets."