genetlink: hold read cb_lock during iteration of genl_fam_idr in genl_bind()

In genl_bind(), currently genl_lock and write cb_lock are taken
for iteration of genl_fam_idr and processing of static values
stored in struct genl_family. Take just read cb_lock for this task
as it is sufficient to guard the idr and the struct against
concurrent genl_register/unregister_family() calls.

This will allow to run genl command processing in genl_rcv() and
mnl_socket_setsockopt(.., NETLINK_ADD_MEMBERSHIP, ..) in parallel.
Reported-by: Vikas Gupta <vikas.gupta@broadcom.com>
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20220825081940.1283335-1-jiri@resnulli.usSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/netlink/genetlink.c

@@ -1362,7 +1362,7 @@ static int genl_bind(struct net *net, int group)
 	unsigned int id;
 	int ret = 0;
 
-	genl_lock_all();
+	down_read(&cb_lock);
 
 	idr_for_each_entry(&genl_fam_idr, family, id) {
 		const struct genl_multicast_group *grp;
@@ -1383,7 +1383,7 @@ static int genl_bind(struct net *net, int group)
 		break;
 	}
 
-	genl_unlock_all();
+	up_read(&cb_lock);
 	return ret;
 }