IB/uverbs: Enable compact representation of uverbs_attr_spec

Downstream patches extend uverbs_attr_spec with new fields.
In order to save space, we move the type and flags fields to
the various attribute flavors contained in the union.

Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>