lib/test_meminit.c: minor test fixes

lib/test_meminit.c: minor test fixes

Fix the following issues in test_meminit.c:
 - |size| in fill_with_garbage_skip() should be signed so that it
   doesn't overflow if it's not aligned on sizeof(*p);
 - fill_with_garbage_skip() should actually skip |skip| bytes;
 - do_kmem_cache_size() should deallocate memory in the RCU case.

Link: http://lkml.kernel.org/r/20190626133135.217355-1-glider@google.com
Fixes: 7e659650cbda ("lib: introduce test_meminit module")
Fixes: 94e8988d91c7 ("lib/test_meminit.c: fix -Wmaybe-uninitialized false positive")
Signed-off-by: Alexander Potapenko <glider@google.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>