When offloading mirroring to gretap or ip6gretap netdevices, an 802.1q
bridge is one of the soft devices permissible in the underlay when
resolving the packet path. After the packet path is resolved to a
particular bridge egress device, flags on packet VLAN determine whether
the egressed packet should be tagged.

The current logic however only ever sets the VLAN tag, never suppresses
it. Thus if there's a VLAN netdevice above the bridge that determines
the packet VLAN, that VLAN is never unset, and mirroring is configured
with VLAN tagging.

Fix by setting the packet VLAN on both branches: set to zero (for unset)
when BRIDGE_VLAN_INFO_UNTAGGED, copy the resolved VLAN (e.g. from bridge
PVID) otherwise.

Fixes: 946a11e7 ("mlxsw: spectrum_span: Allow bridge for gretap mirror")
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>