[media] v4l2-ioctl: clear the reserved field of v4l2_create_buffers

This field was never cleared by the kernel making future extensions
hard to implement. Clear it now.

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