netfilter: fix netfilter_net_init() return

We accidentally return an uninitialized variable.

Fixes: cf56c2f8 ("netfilter: remove old pre-netns era hook api")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>