Introduce driver create and destroy flow methods on the uverbs flow
object.

This allows the driver to get its specific device attributes to match the
underlay specification while still using the generic ib_flow object for
cleanup and code sharing.

The IB object's attributes are set via the ib_set_flow() helper function.

The specific implementation for the given specification is added in
downstream patches.
Signed-off-by: NYishai Hadas <yishaih@mellanox.com>
Signed-off-by: NLeon Romanovsky <leonro@mellanox.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>