SLOB: fix bogus ksize calculation fix

This fixes the previous fix, which was completely wrong on closer
inspection. This version has been manually tested with a user-space
test harness and generates sane values. A nearly identical patch has
been boot-tested.

The problem arose from changing how kmalloc/kfree handled alignment
padding without updating ksize to match. This brings it in sync.
Signed-off-by: NMatt Mackall <mpm@selenic.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/slob.c

@@ -514,9 +514,11 @@ size_t ksize(const void *block)
 		return 0;
 
 	sp = (struct slob_page *)virt_to_page(block);
-	if (slob_page(sp))
-		return (((slob_t *)block - 1)->units - 1) * SLOB_UNIT;
-	else
+	if (slob_page(sp)) {
+		int align = max(ARCH_KMALLOC_MINALIGN, ARCH_SLAB_MINALIGN);
+		unsigned int *m = (unsigned int *)(block - align);
+		return SLOB_UNITS(*m) * SLOB_UNIT;
+	} else
 		return sp->page.private;
 }