[media] media/vivid-osd: fix info leak in ioctl

The vivid_fb_ioctl() code fails to initialize the 16 _reserved bytes of
struct fb_vblank after the ->hcount member. Add an explicit
memset(0) before filling the structure to avoid the info leak.

Signed-off-by: Salva Peiró <speirofr@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
