Two failure patterns are seen randomly when running slub_kunit tests with
CONFIG_SLAB_FREELIST_RANDOM and CONFIG_SLAB_FREELIST_HARDENED enabled.

Pattern 1:
     # test_clobber_zone: pass:1 fail:0 skip:0 total:1
     ok 1 test_clobber_zone
     # test_next_pointer: EXPECTATION FAILED at lib/slub_kunit.c:72
     Expected 3 == slab_errors, but
         slab_errors == 0 (0x0)
     # test_next_pointer: EXPECTATION FAILED at lib/slub_kunit.c:84
     Expected 2 == slab_errors, but
         slab_errors == 0 (0x0)
     # test_next_pointer: pass:0 fail:1 skip:0 total:1
     not ok 2 test_next_pointer

In this case, test_next_pointer() overwrites p[s->offset], but the data
at p[s->offset] is already 0x12.

Pattern 2:
     ok 1 test_clobber_zone
     # test_next_pointer: EXPECTATION FAILED at lib/slub_kunit.c:72
     Expected 3 == slab_errors, but
         slab_errors == 2 (0x2)
     # test_next_pointer: pass:0 fail:1 skip:0 total:1
     not ok 2 test_next_pointer

In this case, p[s->offset] has a value other than 0x12, but one of the
expected failures is nevertheless missing.

Invert data instead of writing a fixed value to corrupt the cache data
structures to fix the problem.

Fixes: 1f9f78b1 ("mm/slub, kunit: add a KUnit test for SLUB debugging functionality")
Cc: Oliver Glitta <glittao@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
CC: Daniel Latypov <dlatypov@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>