netfilter: nft_connlimit: fix possible data race on connection count

nft_connlimit_eval() reads priv->list->count to check if the connection
limit has been exceeded. This value is being read without a lock and can
be modified by a different process. Use READ_ONCE() for correctness.

Fixes: df4a9025 ("netfilter: nf_conncount: merge lookup and add functions")
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Florian Westphal <fw@strlen.de>