media: v4l2-dv-timings.c: fix too strict blanking sanity checks

In the Linux kernel, the following vulnerability has been resolved:

media: v4l2-dv-timings.c: fix too strict blanking sanity checks

Sanity checks were added to verify the v4l2_bt_timings blanking fields

in order to avoid integer overflows when userspace passes weird values.

But that assumed that userspace would correctly fill in the front porch,

backporch and sync values, but sometimes all you know is the total

blanking, which is then assigned to just one of these fields.

And that can fail with these checks.

So instead set a maximum for the total horizontal and vertical

blanking and check that each field remains below that.

That is still sufficient to avoid integer overflows, but it also

allows for more flexibility in how userspace fills in these fields.