bridge: mdb: Allow replace of a host-joined group

Attempts to replace an MDB group membership of the host itself are
currently bounced:

# ip link add name br up type bridge vlan_filtering 1
# bridge mdb replace dev br port br grp 239.0.0.1 vid 2
# bridge mdb replace dev br port br grp 239.0.0.1 vid 2
Error: bridge: Group is already joined by host.

A similar operation done on a member port would succeed. Ignore the check
for replacement of host group memberships as well.

The bit of code that this enables is br_multicast_host_join(), which, for
already-joined groups only refreshes the MC group expiration timer, which
is desirable; and a userspace notification, also desirable.

Change a selftest that exercises this code path from expecting a rejection
to expecting a pass. The rest of MDB selftests pass without modification.

Signed-off-by: Petr Machata <[e-mail]>
Reviewed-by: Ido Schimmel <[e-mail]>
Acked-by: Nikolay Aleksandrov <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/bridge/br_mdb.c

···
 
 	/* host join */
 	if (!port) {
-		if (mp->host_joined) {
+		if (mp->host_joined && !(cfg->nlflags & NLM_F_REPLACE)) {
 			NL_SET_ERR_MSG_MOD(extack, "Group is already joined by host");
 			return -EEXIST;
 		}

tools/testing/selftests/net/forwarding/bridge_mdb.sh

···
 	check_err $? "Failed to add $name host entry"
 
 	bridge mdb replace dev br0 port br0 grp $grp $state vid 10 &> /dev/null
-	check_fail $? "Managed to replace $name host entry"
+	check_err $? "Failed to replace $name host entry"
 
 	bridge mdb del dev br0 port br0 grp $grp $state vid 10
 	bridge mdb get dev br0 grp $grp vid 10 &> /dev/null