IB/uverbs: Add support for flow counters

The struct ib_uverbs_flow_spec_action_count associates a counters object
with the flow.

Post this association the flow counters can be read via the counters
object.

Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Raed Salem <raeds@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>