bridge: mcast: Fail MDB get request on empty entry

When user space deletes a port from an MDB entry, the port is removed
synchronously. If this was the last port in the entry and the entry is
not joined by the host itself, then the entry is scheduled for deletion
via a timer.

The above means that it is possible for the MDB get netlink request to
retrieve an empty entry which is scheduled for deletion. This is
problematic as after deleting the last port in an entry, user space
cannot rely on a non-zero return code from the MDB get request as an
indication that the port was successfully removed.

Fix by returning an error when the entry's port list is empty and the
entry is not joined by the host.

Fixes: 68b380a3 ("bridge: mcast: Add MDB get support")
Reported-by: Jamie Bainbridge <jamie.bainbridge@gmail.com>
Closes: https://lore.kernel.org/netdev/c92569919307749f879b9482b0f3e125b7d9d2e3.1726480066.git.jamie.bainbridge@gmail.com/Tested-by: Jamie Bainbridge <jamie.bainbridge@gmail.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20240929123640.558525-1-idosch@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/bridge/br_mdb.c

@@ -1674,7 +1674,7 @@ int br_mdb_get(struct net_device *dev, struct nlattr *tb[], u32 portid, u32 seq,
 	spin_lock_bh(&br->multicast_lock);
 
 	mp = br_mdb_ip_get(br, &group);
-	if (!mp) {
+	if (!mp || (!mp->ports && !mp->host_joined)) {
 		NL_SET_ERR_MSG_MOD(extack, "MDB entry not found");
 		err = -ENOENT;
 		goto unlock;