kfence: add sysfs interface to disable kfence for selected slabs.

kfence: add sysfs interface to disable kfence for selected slabs.

By default kfence allocation can happen for any slab object, whose size is
up to PAGE_SIZE, as long as that allocation is the first allocation after
expiration of kfence sample interval.  But in certain debugging scenarios
we may be interested in debugging corruptions involving some specific slub
objects like dentry or ext4_* etc.  In such cases limiting kfence for
allocations involving only specific slub objects will increase the
probablity of catching the issue since kfence pool will not be consumed by
other slab objects.

This patch introduces a sysfs interface
'/sys/kernel/slab/<name>/skip_kfence' to disable kfence for specific
slabs.  Having the interface work in this way does not impact
current/default behavior of kfence and allows us to use kfence for
specific slabs (when needed) as well.  The decision to skip/use kfence is
taken depending on whether kmem_cache.flags has (newly introduced)
SLAB_SKIP_KFENCE flag set or not.

Link: https://lkml.kernel.org/r/20220814195353.2540848-1-imran.f.khan@oracle.com
Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Marco Elver <elver@google.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.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>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>