slab: make size_index[] array u8

All those small numbers are reverse indexes into kmalloc caches array
and can't be negative.

On x86_64 "unsigned int = fls()" can drop CDQE instruction:

	add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-2 (-2)
	Function                                     old     new   delta
	kmalloc_slab                                 101      99      -2

Link: http://lkml.kernel.org/r/20180305200730.15812-7-adobriyan@gmail.com
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slab_common.c

@@ -971,7 +971,7 @@ EXPORT_SYMBOL(kmalloc_dma_caches);
  * of two cache sizes there. The size of larger slabs can be determined using
  * fls.
  */
-static s8 size_index[24] __ro_after_init = {
+static u8 size_index[24] __ro_after_init = {
 	3,	/* 8 */
 	4,	/* 16 */
 	5,	/* 24 */
@@ -1009,7 +1009,7 @@ static inline int size_index_elem(size_t bytes)
  */
 struct kmem_cache *kmalloc_slab(size_t size, gfp_t flags)
 {
-	int index;
+	unsigned int index;
 
 	if (unlikely(size > KMALLOC_MAX_SIZE)) {
 		WARN_ON_ONCE(!(flags & __GFP_NOWARN));