qbt_handler: Memset userspace struct to zero

Explicitly memset struct to zero before sending
to userspace. This is to prevent padded bytes
from possibly leaking kernel memory contents to
userspace.

Change-Id: Id3e873164430c34195344884b2899e61aa060dfc
Signed-off-by: Abir Ghosh <abirg@codeaurora.org>