net: bridge: add per-port group_fwd_mask with less restrictions

We need to be able to transparently forward most link-local frames via
tunnels (e.g. vxlan, qinq). Currently the bridge's group_fwd_mask has a
mask which restricts the forwarding of STP and LACP, but we need to be able
to forward these over tunnels and control that forwarding on a per-port
basis thus add a new per-port group_fwd_mask option which only disallows
mac pause frames to be forwarded (they're always dropped anyway).
The patch does not change the current default situation - all of the others
are still restricted unless configured for forwarding.
We have successfully tested this patch with LACP and STP forwarding over
VxLAN and qinq tunnels.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>