IB/uverbs: Add read counters support

This patch exposes the read counters verb to user space applications.  By
that verb the user can read the hardware counters which are associated
with the counters object.

The application needs to provide a sufficient memory to hold the
statistics.

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>