slub: make slab_index() return unsigned int

slab_index() returns index of an object within a slab which is at most
u15 (or u16?).

Iterators additionally guarantee that "p >= addr".

Link: http://lkml.kernel.org/r/20180305200730.15812-22-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/slub.c

@@ -311,7 +311,7 @@ static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
 		__p += (__s)->size, __idx++)
 
 /* Determine object index from a given position */
-static inline int slab_index(void *p, struct kmem_cache *s, void *addr)
+static inline unsigned int slab_index(void *p, struct kmem_cache *s, void *addr)
 {
 	return (p - addr) / s->size;
 }