mm: move obj_to_index to include/linux/slab_def.h

mm: move obj_to_index to include/linux/slab_def.h

While with SLUB we can actually preassign tags for caches with contructors
and store them in pointers in the freelist, SLAB doesn't allow that since
the freelist is stored as an array of indexes, so there are no pointers to
store the tags.

Instead we compute the tag twice, once when a slab is created before
calling the constructor and then again each time when an object is
allocated with kmalloc.  Tag is computed simply by taking the lowest byte
of the index that corresponds to the object.  However in kasan_kmalloc we
only have access to the objects pointer, so we need a way to find out
which index this object corresponds to.

This patch moves obj_to_index from slab.c to include/linux/slab_def.h to
be reused by KASAN.

Link: http://lkml.kernel.org/r/c02cd9e574cfd93858e43ac94b05e38f891fef64.1544099024.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>