media: vivid: fix buffer overwrite when using > 32 buffers

The maximum number of buffers that can be requested was increased to
64 for the video capture queue. But video capture used a must_blank
array that was still sized for 32 (VIDEO_MAX_FRAME). This caused an
out-of-bounds write when using buffer indices >= 32.

Create a new define MAX_VID_CAP_BUFFERS that is used to access the
must_blank array and set max_num_buffers for the video capture queue.

This solves a crash reported by:

	https://bugzilla.kernel.org/show_bug.cgi?id=219258



Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Fixes: cea70ed4 ("media: test-drivers: vivid: Increase max supported buffers for capture queues")
Cc: stable@vger.kernel.org