mm/slub: Consider kfence case for get_orig_size()

When 'orig_size' of kmalloc object is enabled by debug option, it
should either contains the actual requested size or the cache's
'object_size'.

But it's not true if that object is a kfence-allocated one, and the
data at 'orig_size' offset of metadata could be zero or other values.
This is not a big issue for current 'orig_size' usage, as init_object()
and check_object() during alloc/free process will be skipped for kfence
addresses. But it could cause trouble for other usage in future.

Use the existing kfence helper kfence_ksize() which can return the
real original request size.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>