IB/core: Avoid leakage from kernel to user space

commit 377b513485fd885dea1083a9a5430df65b35e048 upstream.

Clear the reserved field of struct ib_uverbs_async_event_desc which is
copied to user space.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>