mm/slab: document kfree() as allowed for kmem_cache_alloc() objects

This will make it easier to free objects in situations when they can
come from either kmalloc() or kmem_cache_alloc(), and also allow
kfree_rcu() for freeing objects from kmem_cache_alloc().

For the SLAB and SLUB allocators this was always possible so with SLOB
gone, we can document it as supported.

Signed-off-by: Vlastimil Babka <[e-mail]>
Reviewed-by: Mike Rapoport (IBM) <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: "Paul E. McKenney" <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Cc: Neeraj Upadhyay <[e-mail]>
Cc: Josh Triplett <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Cc: Lai Jiangshan <[e-mail]>
Cc: Joel Fernandes <[e-mail]>

Documentation/core-api/memory-allocation.rst

···
 After the cache is created kmem_cache_alloc() and its convenience
 wrappers can allocate memory from that cache.
 
-When the allocated memory is no longer needed it must be freed. You can
-use kvfree() for the memory allocated with `kmalloc`, `vmalloc` and
-`kvmalloc`. The slab caches should be freed with kmem_cache_free(). And
-don't forget to destroy the cache with kmem_cache_destroy().
+When the allocated memory is no longer needed it must be freed.
+
+Objects allocated by `kmalloc` can be freed by `kfree` or `kvfree`. Objects
+allocated by `kmem_cache_alloc` can be freed with `kmem_cache_free`, `kfree`
+or `kvfree`, where the latter two might be more convenient thanks to not
+needing the kmem_cache pointer.
+
+The same rules apply to _bulk and _rcu flavors of freeing functions.
+
+Memory allocated by `vmalloc` can be freed with `vfree` or `kvfree`.
+Memory allocated by `kvmalloc` can be freed with `kvfree`.
+Caches created by `kmem_cache_create` should be freed with
+`kmem_cache_destroy` only after freeing all the allocated objects first.

include/linux/rcupdate.h

···
  * either fall back to use of call_rcu() or rearrange the structure to
  * position the rcu_head structure into the first 4096 bytes.
  *
- * Note that the allowable offset might decrease in the future, for example,
- * to allow something like kmem_cache_free_rcu().
+ * The object to be freed can be allocated either by kmalloc() or
+ * kmem_cache_alloc().
+ *
+ * Note that the allowable offset might decrease in the future.
  *
  * The BUILD_BUG_ON check must not involve any function calls, hence the
  * checks are done in macros here.

mm/slab_common.c

···
 
 /**
  * kfree - free previously allocated memory
- * @object: pointer returned by kmalloc.
+ * @object: pointer returned by kmalloc() or kmem_cache_alloc()
  *
  * If @object is NULL, no operation is performed.
- *
- * Don't free memory not originally allocated by kmalloc()
- * or you will run into trouble.
  */
 void kfree(const void *object)
 {