netfilter: nft_counter: fix erroneous return values

The nft_counter_init() and nft_counter_clone() functions should return
negative error value -ENOMEM instead of positive ENOMEM.

Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>