netfilter: nft_counter: remove wrong __percpu of nft_counter_resest()'s arg

nft_counter_rest() has its first argument declared as
	struct nft_counter_percpu_priv __percpu *priv
but this structure is not percpu (it only countains
a member 'counter' which is, correctly, a pointer to a
percpu struct nft_counter).

So, remove the '__percpu' from the argument's declaration.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>