IB/uverbs: Add create/destroy counters support

User space application which uses counters functionality, is expected to
allocate/release the counters resources by calling create/destroy verbs
and in turn get a unique handle that can be used to attach the counters to
its counted type.

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>