drm/xe/tests: Fix the shrinker test compiler warnings.

The xe_bo_shrink_kunit test has an uninitialized value and illegal
integer size conversions on 32-bit. Fix.

v2:
- Use div64_u64 to ensure the u64 division compiles everywhere. (Matt Auld)

Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/20240913195649.GA61514@thelio-3990X/
Fixes: 5a90b60db5 ("drm/xe: Add a xe_bo subtest for shrinking / swapping")
Cc: dri-devel@lists.freedesktop.org
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com> #v1
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241004141121.186177-1-thomas.hellstrom@linux.intel.com

drivers/gpu/drm/xe/tests/xe_bo.c

@@ -7,6 +7,7 @@
 #include <kunit/visibility.h>
 
 #include <linux/iosys-map.h>
+#include <linux/math64.h>
 #include <linux/random.h>
 #include <linux/swap.h>
 
@@ -440,7 +441,7 @@ static int shrink_test_run_device(struct xe_device *xe)
 	LIST_HEAD(bos);
 	struct xe_bo_link *link, *next;
 	struct sysinfo si;
-	size_t ram, ram_and_swap, purgeable, alloced, to_alloc, limit;
+	u64 ram, ram_and_swap, purgeable = 0, alloced, to_alloc, limit;
 	unsigned int interrupted = 0, successful = 0, count = 0;
 	struct rnd_state prng;
 	u64 rand_seed;
@@ -469,7 +470,7 @@ static int shrink_test_run_device(struct xe_device *xe)
 	ram_and_swap = ram + get_nr_swap_pages() * PAGE_SIZE;
 	if (to_alloc > ram_and_swap)
 		purgeable = to_alloc - ram_and_swap;
-	purgeable += purgeable / 5;
+	purgeable += div64_u64(purgeable, 5);
 
 	kunit_info(test, "Free ram is %lu bytes. Will allocate twice of that.\n",
 		   (unsigned long)ram);