IB/core: Add legacy driver's user-data

In this phase, we don't want to change all the drivers to use
flexible driver's specific attributes. Therefore, we add two default
attributes: UHW_IN and UHW_OUT. These attributes are optional in some
methods and they encode the driver specific command data. We add
a function that extract this data and creates the legacy udata over
it.

Driver's data should start from UVERBS_UDATA_DRIVER_DATA_FLAG. This
turns on the first bit of the namespace, indicating this attribute
belongs to the driver's namespace.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>