drm/msm: Handle possible overflow in rsp len calculation

If rsp_off, rsp_mem_sz, and len are all zero we could end getting past
the size checks, but msm_context_rsp_noshadow() would still return NULL.

But we should never end up with len==0.  Fix rsp length calcs to use the
overflow handling helpers, and add an assert.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Part-of: <https://gitlab.freedesktop.org/virgl/virglrenderer/-/merge_requests/795>
1 file changed