IB/core: Change completion channel to use the reworked objects schema

This patch adds the standard fd based type - completion_channel.
The completion_channel is now prefixed with ib_uobject, similarly
to the rest of the uobjects.
This requires a few changes:
(1) We define a new completion channel fd based object type.
(2) completion_event and async_event are now two different types.
    This means they use different fops.
(3) We release the completion_channel exactly as we release other
    idr based objects.
(4) Since ib_uobjects are already kref-ed, we only add the kref to the
    async event.

A fd object requires filling out several parameters. Its op pointer
should point to uverbs_fd_ops and its size should be at least the
size if ib_uobject. We use a macro to make the type declaration
easier.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>