switchdev: introduce switchdev add/del obj ops

Like switchdev attr get/set, add new switchdev obj add/del.  switchdev objs
will be things like VLANs or FIB entries, so add/del fits better for
objects than get/set used for attributes.

Use same two-phase prepare-commit transaction model as in attr set.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Acked-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>