mlxsw: spectrum_span: Ignore VLAN entries not used by the bridge in mirroring

Only VLAN entries installed on the bridge device itself should be
considered when checking whether a packet with a specific VLAN can be
mirrored via a bridge device. VLAN entries only used to keep context
(i.e., entries with 'BRIDGE_VLAN_INFO_BRENTRY' unset) should be ignored.

Fix this by preventing mirroring when the VLAN entry does not have the
'BRIDGE_VLAN_INFO_BRENTRY' flag set.

Fixes: ddaff504 ("mlxsw: spectrum: remove guards against !BRIDGE_VLAN_INFO_BRENTRY")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>