kernel: backport switchdev fix for bridge in bridge configurations

kernel: backport switchdev fix for bridge in bridge configurations

This patch fixes the forwarding behavior of bridge in bridge
configurations with DSA.

Without it, the configuration of the upper bridge might overwrite
settings of the lower bridge. For example, a vlan-aware bridge
with DSA interfaces in it might be offloaded to the DSA hardware. If the
bridge interface itself gets slave of a different bridge without vlan
filtering, the vlan filtering setting of the lower bridge is overwritten
by the upper bridge, which results in an incorrect hardware
configuration.

This was backported from kernel 5.7.

Ref: https://lore.kernel.org/netdev/20191222192235.GK25745@shell.armlinux.org.uk/
Fixes: FS#3996
Signed-off-by: Fabian Bläse <fabian@blaese.de>