netfilter: ipset: Return ipset error instead of bool

Statement ret = func1() || func2() returns 0 when both func1()
and func2() return 0, or 1 if func1() or func2() returns non-zero.

However in our case func1() and func2() returns error code on
failure, so it seems good to propagate such error codes, rather
than returning 1 in case of failure.

Signed-off-by: Sergey Popovich <popovich_sergei@mail.ua>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>