netfilter: nft_socket: fix sk refcount leaks

We must put 'sk' reference before returning.

Fixes: 039b1f4f ("netfilter: nft_socket: fix erroneous socket assignment")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>