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>

lib/test_meminit.c

@@ -38,15 +38,14 @@ static int __init count_nonzero_bytes(void *ptr, size_t size)
 }
 
 /* Fill a buffer with garbage, skipping |skip| first bytes. */
-static void __init fill_with_garbage_skip(void *ptr, size_t size, size_t skip)
+static void __init fill_with_garbage_skip(void *ptr, int size, size_t skip)
 {
-	unsigned int *p = (unsigned int *)ptr;
+	unsigned int *p = (unsigned int *)((char *)ptr + skip);
 	int i = 0;
 
-	if (skip) {
-		WARN_ON(skip > size);
-		p += skip;
-	}
+	WARN_ON(skip > size);
+	size -= skip;
+
 	while (size >= sizeof(*p)) {
 		p[i] = GARBAGE_INT;
 		i++;
@@ -227,6 +226,7 @@ static int __init do_kmem_cache_size(size_t size, bool want_ctor,
 		if (buf_copy)
 			memcpy(buf_copy, buf, size);
 
+		kmem_cache_free(c, buf);
 		/*
 		 * Check that |buf| is intact after kmem_cache_free().
 		 * |want_zero| is false, because we wrote garbage to