mm/slub: fix help comment of SLUB_DEBUG

Since commit ab4d5ed5 ("slub: Enable sysfs support for
!CONFIG_SLUB_DEBUG"), disabling SLUB_DEBUG no longer disables sysfs
support completely, so fix the description. Also update the path to
/sys/kernel/slab.

Signed-off-by: Vernon Yang <vernon2gm@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>