netfilter: nfnetlink: silence warning if CONFIG_PROVE_RCU isn't set

Since commit c14b78e7 ("netfilter:
nfnetlink: add mutex per subsystem") building nefnetlink.o without
CONFIG_PROVE_RCU set, triggers this GCC warning:
    net/netfilter/nfnetlink.c:65:22: warning: ‘nfnl_get_lock’ defined but not used [-Wunused-function]

The cause of that warning is, in short, that rcu_lockdep_assert()
compiles away if CONFIG_PROVE_RCU is not set. Silence this warning by
open coding nfnl_get_lock() in the sole place it was called, which
allows to remove that function.
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nfnetlink.c

@@ -62,11 +62,6 @@ void nfnl_unlock(__u8 subsys_id)
 }
 EXPORT_SYMBOL_GPL(nfnl_unlock);
 
-static struct mutex *nfnl_get_lock(__u8 subsys_id)
-{
-	return &table[subsys_id].mutex;
-}
-
 int nfnetlink_subsys_register(const struct nfnetlink_subsystem *n)
 {
 	nfnl_lock(n->subsys_id);
@@ -199,7 +194,7 @@ static int nfnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 			rcu_read_unlock();
 			nfnl_lock(subsys_id);
 			if (rcu_dereference_protected(table[subsys_id].subsys,
-				lockdep_is_held(nfnl_get_lock(subsys_id))) != ss ||
+				lockdep_is_held(&table[subsys_id].mutex)) != ss ||
 			    nfnetlink_find_client(type, ss) != nc)
 				err = -EAGAIN;
 			else if (nc->call)