netfilter: nfnetlink: don't probe module if it exists

nfnetlink_bind request_module()s all the time as nfnetlink_get_subsys()
shifts the argument by 8 to obtain the subsys id.

So using type instead of type << 8 always returns NULL.

Fixes: 03292745

 ("netlink: add nlk->netlink_bind hook for module auto-loading")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>