netfilter: rename local nf_hook_list to hook_list

085db2c0 ("netfilter: Per network namespace netfilter hooks.") introduced a
new nf_hook_list that is global, so let's avoid this overlap.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>