media: v4l2-ioctl.c: zero reserved fields for S/TRY_FMT

v4l2_vbi_format, v4l2_sliced_vbi_format and v4l2_sdr_format
have a reserved array at the end that should be zeroed by drivers
as per the V4L2 spec. Older drivers often do not do this, so just
handle this in the core.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>