switchdev: Adding complete operation to deferred switchdev ops

When using switchdev deferred operation (SWITCHDEV_F_DEFER), the operation
is executed in different context and the application doesn't have any way
to get the operation real status.

Adding a completion callback fixes that. This patch adds fields to
switchdev_attr and switchdev_obj "complete_priv" field which is used by
the "complete" callback.

Application can set a complete function which will be called once the
operation executed.

Signed-off-by: Elad Raz <eladr@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>