netlink: don't call ->netlink_bind with table lock held

When I added support to allow generic netlink multicast groups to be
restricted to subscribers with CAP_NET_ADMIN I was unaware that a
genl_bind implementation already existed in the past.

It was reverted due to ABBA deadlock:

1. ->netlink_bind gets called with the table lock held.
2. genetlink bind callback is invoked, it grabs the genl lock.

But when a new genl subsystem is (un)registered, these two locks are
taken in reverse order.

One solution would be to revert again and add a comment in genl
referring 1e82a62fec613, "genetlink: remove genl_bind").

This would need a second change in mptcp to not expose the raw token
value anymore, e.g. by hashing the token with a secret key so userspace
can still associate subflow events with the correct mptcp connection.

However, Paolo Abeni reminded me to double-check why the netlink table is
locked in the first place.

I can't find one. netlink_bind() is already called without this lock
when userspace joins a group via NETLINK_ADD_MEMBERSHIP setsockopt.
Same holds for the netlink_unbind operation.

Digging through the history, commit f773608026ee1
("netlink: access nlk groups safely in netlink bind and getname")
expanded the lock scope.

commit 3a20773beeeeade ("net: netlink: cap max groups which will be considered in netlink_bind()")
... removed the nlk->ngroups access that the lock scope
extension was all about.

Reduce the lock scope again and always call ->netlink_bind without
the table lock.

The Fixes tag should be vs. the patch mentioned in the link below,
but that one got squash-merged into the patch that came earlier in the
series.

Fixes: 4d54cc32112d8d ("mptcp: avoid lock_fast usage in accept path")
Link: https://lore.kernel.org/mptcp/[e-mail]/T/#u
Cc: Cong Wang <[e-mail]>
Cc: Xin Long <[e-mail]>
Cc: Johannes Berg <[e-mail]>
Cc: Sean Tranchetti <[e-mail]>
Cc: Paolo Abeni <[e-mail]>
Cc: Pablo Neira Ayuso <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/netlink/af_netlink.c

···
 			return -EINVAL;
 	}
 
-	netlink_lock_table();
 	if (nlk->netlink_bind && groups) {
 		int group;
 
···
 			if (!err)
 				continue;
 			netlink_undo_bind(group, groups, sk);
-			goto unlock;
+			return err;
 		}
 	}
 
 	/* No need for barriers here as we return to user-space without
 	 * using any of the bound attributes.
 	 */
+	netlink_lock_table();
 	if (!bound) {
 		err = nladdr->nl_pid ?
 			netlink_insert(sk, nladdr->nl_pid) :