drm/msm/gem: prevent integer overflow in msm_ioctl_gem_submit()

The "submit->cmd[i].size" and "submit->cmd[i].offset" variables are u32
values that come from the user via the submit_lookup_cmds() function.
This addition could lead to an integer wrapping bug so use size_add()
to prevent that.

Fixes: 19872533 ("drm/msm: fix cmdstream size check")
Cc: stable@vger.kernel.org
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/624696/


Signed-off-by: Rob Clark <robdclark@chromium.org>