[media] vb2: Fix compilation breakage when !CONFIG_BUG

Commit 77a3c6fd ("[media] vb2: Don't WARN when v4l2_buffer.bytesused
is 0 for multiplanar buffers") uses the __WARN() macro which isn't
defined when CONFIG_BUG isn't set. This introduces a compilation
breakage. Fix it by using WARN_ON() instead.

The commit was also broken in that it merged v1 of the patch while a new
v2 version had been submitted, reviewed and acked. Fix it by
incorporating the changes from v1 to v2.

Fixes: 77a3c6fd ("[media] vb2: Don't WARN when v4l2_buffer.bytesused is 0 for multiplanar buffers")

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>