kernel: bridge: Implement MLD Querier wake-up calls / Android bug workaround

Implement a configurable MLD Querier wake-up calls "feature" which
works around a widely spread Android bug in connection with IGMP/MLD
snooping.

Currently there are mobile devices (e.g. Android) which are not able
to receive and respond to MLD Queries reliably because the Wifi driver
filters a lot of ICMPv6 when the device is asleep - including
MLD. This in turn breaks IPv6 communication when MLD Snooping is
enabled. However there is one ICMPv6 type which is allowed to pass and
which can be used to wake up the mobile device: ICMPv6 Echo Requests.

If this bridge is the selected MLD Querier then setting
"multicast_wakeupcall" to a number n greater than 0 will send n
ICMPv6 Echo Requests to each host behind this port to wake
them up with each MLD Query. Upon receiving a matching ICMPv6 Echo
Reply an MLD Query with a unicast ethernet destination will be sent
to the specific host(s).

Link: https://issuetracker.google.com/issues/149630944
Link: https://github.com/freifunk-gluon/gluon/issues/1832



Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>