IB/core: Nullify ib_uobject during allocation

Currently, we initialize all fields of ib_uobject straight after
allocation. Therefore, a kmalloc was sufficient. Since ib_uobject
could be embedded in a type specific structure, we nullify it to
spare programmer errors.

Fixes: 38321256 ('IB/core: Add support for idr types')
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>