SWITCHDEV_ATTR_ID_BRIDGE_VLAN_FILTERING is notified by the bridge from
two places:
- nbp_vlan_init(), during bridge port creation
- br_vlan_filter_toggle(), during a netlink/sysfs/ioctl change requested
  by user space

If a switchdev driver uses br_vlan_enabled(br_dev) inside its handler
for the SWITCHDEV_ATTR_ID_BRIDGE_VLAN_FILTERING attribute notifier,
different things will be seen depending on whether the bridge calls from
the first path or the second:
- in nbp_vlan_init(), br_vlan_enabled() reflects the current state of
  the bridge
- in br_vlan_filter_toggle(), br_vlan_enabled() reflects the past state
  of the bridge

This can lead in some cases to complications in driver implementation,
which can be avoided if these could reliably use br_vlan_enabled().

Nothing seems to depend on this behavior, and it seems overall more
straightforward for br_vlan_enabled() to return the proper value even
during the SWITCHDEV_ATTR_ID_BRIDGE_VLAN_FILTERING notifier, so
temporarily enable the bridge option, then revert it if the switchdev
notifier failed.

Cc: Roopa Prabhu <roopa@nvidia.com>
Cc: Nikolay Aleksandrov <nikolay@nvidia.com>
Cc: Ido Schimmel <idosch@nvidia.com>
Cc: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>