IB/core: Don't use is_async in event files to infer events size

Previously, we inferred the events size in ib_uverbs_event_read by
using the is_async flag. Instead of that, we pass the event size
directly.

Fixes: 1e7710f3 ('IB/core: Change completion channel to use the reworked objects schema')
Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>