drm/xe: fix bounds checking for 'len' in xe_engine_create_ioctl

There's this shared machine running xe.ko and I often log in to see my
tmux corrupted by messages such as:

    usercopy: Kernel memory overwrite attempt detected to wrapped address (offset 0, size 18446660151965198754)!

I also sometimes see:

    kernel BUG at mm/usercopy.c:102!

Someone is running a program that's definitely submitting random
numbers to this ioctl. If you pass width=65535 and
num_placements=32769 then you get a negative 'len', which avoids the
EINVAL check, leading to the bug.

Switch 'len' to u32. It is the result of the multiplication of two u16
numbers, so it won't be able to overflow back into smaller numbers as
an u32.

v2: Make len u32 instead of checking for <=0 (José).
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20230626212221.136640-1-paulo.r.zanoni@intel.comSigned-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_engine.c

@@ -522,7 +522,7 @@ int xe_engine_create_ioctl(struct drm_device *dev, void *data,
 	struct xe_engine *e = NULL;
 	u32 logical_mask;
 	u32 id;
-	int len;
+	u32 len;
 	int err;
 
 	if (XE_IOCTL_ERR(xe, args->flags) ||