vxlan: Add extack to switchdev operations

There are four sources of VXLAN switchdev notifier calls:

- the changelink() link operation, which already supports extack,
- ndo_fdb_add() which got extack support in a previous patch,
- FDB updates due to packet forwarding,
- and vxlan_fdb_replay().

Extend vxlan_fdb_switchdev_call_notifiers() to include extack in the
switchdev message that it sends, and propagate the argument upwards to
the callers. For the first two cases, pass in the extack gotten through
the operation. For case #3, pass in NULL.

To cover the last case, extend vxlan_fdb_replay() to take extack
argument, which might come from whatever operation necessitated the FDB
replay.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>