IB/core: Change idr objects to use the new schema

This changes only the handlers which deals with idr based objects to
use the new idr allocation, fetching and destruction schema.
This patch consists of the following changes:
(1) Allocation, fetching and destruction is done via idr ops.
(2) Context initializing and release is done through
    uverbs_initialize_ucontext and uverbs_cleanup_ucontext.
(3) Ditching the live flag. Mostly, this is pretty straight
    forward. The only place that is a bit trickier is in
    ib_uverbs_open_qp. Commit [1] added code to check whether
    the uobject is already live and initialized. This mostly
    happens because of a race between open_qp and events.
    We delayed assigning the uobject's pointer in order to
    eliminate this race without using the live variable.

[1] commit a040f95d


	("IB/core: Fix XRC race condition in ib_uverbs_open_qp")

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