drm/msm: fix an integer overflow test

We recently added an integer overflow check but it needs an additional
tweak to work properly on 32 bit systems.

The problem is that we're doing the right hand side of the assignment as
type unsigned long so the max it will have an integer overflow instead
of being larger than SIZE_MAX.  That means the "sz > SIZE_MAX" condition
is never true even on 32 bit systems.  We need to first cast it to u64
and then do the math.

Fixes: 4a630fadbb29 ("drm/msm: Fix potential buffer overflow issue")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>