netfilter: nftables: comment indirect serialization of commit_mutex with rtnl_mutex

Add an explicit comment in the code to describe the indirect
serialization of the holders of the commit_mutex with the rtnl_mutex.
Commit 90d2723c ("netfilter: nf_tables: do not hold reference on
netdevice from preparation phase") already describes this, but a comment
in this case is better for reference.
Reported-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -1723,6 +1723,10 @@ static struct nft_hook *nft_netdev_hook_alloc(struct net *net,
 	}
 
 	nla_strlcpy(ifname, attr, IFNAMSIZ);
+	/* nf_tables_netdev_event() is called under rtnl_mutex, this is
+	 * indirectly serializing all the other holders of the commit_mutex with
+	 * the rtnl_mutex.
+	 */
 	dev = __dev_get_by_name(net, ifname);
 	if (!dev) {
 		err = -ENOENT;