IB/core: Introduce counters object and its create/destroy

A verbs application may need to get statistics and info on various aspects
of a verb object (e.g. Flow, QP, ...), in general case the application
will state which object's counters its interested in (we refer to this
action as attach), bind this new counters object to the appropriate verb
object and on later stage read their values using the counters object.

This series introduces a general API for counters object that may
accumulate any ib object counters type, bound and read on demand.

Counters instance is allocated on an IB context and belongs to that
context.  Upon successful creation the counters can be bound to a verbs
object so that hardware counter instances can be created and read.

Downstream patches in this series will introduce the attach, bind and the
read functionality.

Counters instance can be de-allocated, upon successful destruction the
related hardware resources are released.

Prior to destroy call the user must first make sure that the counters is
not being used by any IB object, e.g. not attached to any of its counted
type otherwise an EBUSY error is invoked.

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>