mm: zbud: initialize object to 0 on GFP_ZERO

zbud_alloc if returns free object from pool must also
initialize it to 0 when asked to do so. The same is
already taken care if a fresh object is allocated.

CRs-fixed: 979234
Change-Id: Id171edf131df321385fcdcd7660d06da97689e3e
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>