mm, kfence: describe @slab parameter in __kfence_obj_info()

Sphinx reports kernel-doc warning:

WARNING: ./include/linux/kfence.h:220 function parameter 'slab' not described in '__kfence_obj_info'

Fix it by describing @slab parameter.

Link: https://lkml.kernel.org/r/20251219014006.16328-6-bagasdotme@gmail.com
Fixes: 2dfe63e61c ("mm, kfence: support kmem_dump_obj() for KFENCE objects")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: Marco Elver <elver@google.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Acked-by: Harry Yoo <harry.yoo@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/kfence.h

@@ -211,6 +211,7 @@ struct kmem_obj_info;
  * __kfence_obj_info() - fill kmem_obj_info struct
  * @kpp: kmem_obj_info to be filled
  * @object: the object
+ * @slab: the slab
  *
  * Return:
  * * false - not a KFENCE object