mm, slub, kasan: enable user tracking by default with KASAN=y

It's recommended to have slub's user tracking enabled with CONFIG_KASAN,
because:

a) User tracking disables slab merging which improves
    detecting out-of-bounds accesses.
b) User tracking metadata acts as redzone which also improves
    detecting out-of-bounds accesses.
c) User tracking provides additional information about object.
    This information helps to understand bugs.

Currently it is not enabled by default.  Besides recompiling the kernel
with KASAN and reinstalling it, user also have to change the boot cmdline,
which is not very handy.

Enable slub user tracking by default with KASAN=y, since there is no good
reason to not do this.

[akpm@linux-foundation.org: little fixes, per David]
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: 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>