slub: make ->offset unsigned int

->offset is free pointer offset from the start of the object, can't be
negative.

Link: http://lkml.kernel.org/r/20180305200730.15812-16-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>

include/linux/slub_def.h

@@ -86,7 +86,7 @@ struct kmem_cache {
 	unsigned long min_partial;
 	int size;		/* The size of an object including meta data */
 	int object_size;	/* The size of an object without meta data */
-	int offset;		/* Free pointer offset. */
+	unsigned int offset;	/* Free pointer offset. */
 #ifdef CONFIG_SLUB_CPU_PARTIAL
 	/* Number of per cpu partial objects to keep around */
 	unsigned int cpu_partial;