base: genlock: Zero a structure before sending it to user space

In the handler for GENLOCK_IOC_EXPORT zero the entire genlock_lock
structure before passing it to the user.

CRs-fixed: 552303
Change-Id: Ic0dedbad257bf0a448d0bf67a14a3932b7925bfc
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>