nouveau/u_memcpya: fix NULL vs error pointer bug

The u_memcpya() function is supposed to return error pointers on
error. Returning NULL will lead to an Oops.

Fixes: e3885f71 ("nouveau/u_memcpya: use vmemdup_user")
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/10fd258b-466f-4c5b-9d48-fe61a3f21424@moroto.mountain

drivers/gpu/drm/nouveau/nouveau_drv.h

@@ -193,7 +193,7 @@ u_memcpya(uint64_t user, unsigned int nmemb, unsigned int size)
 	size_t bytes;
 
 	if (unlikely(check_mul_overflow(nmemb, size, &bytes)))
-		return NULL;
+		return ERR_PTR(-EOVERFLOW);
 	return vmemdup_user(userptr, bytes);
 }