netfilter: nat: fix cmp return value

The comparator works like memcmp, i.e. 0 means objects are equal.
In other words, when objects are distinct they are treated as identical,
when they are distinct they are allegedly the same.

The first case is rare (distinct objects are unlikely to get hashed to
same bucket).

The second case results in unneeded port conflict resolutions attempts.

Fixes: 870190a9 ("netfilter: nat: convert nat bysrc hash to rhashtable")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>