slab: document kzfree() zeroing behavior

As suggested by Alan Cox, document the fact that kzfree() can zero out a great
deal more memory than the what the user requested from kmalloc().

Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>

mm/util.c

diff --git a/mm/util.c b/mm/util.c
index 55bef16..e79572b 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -166,6 +166,10 @@
  *
  * The memory of the object @p points to is zeroed before freed.
  * If @p is %NULL, kzfree() does nothing.
+ *
+ * Note: this function zeroes the whole allocated buffer which can be a good
+ * deal bigger than the requested buffer size passed to kmalloc(). So be
+ * careful when using this function in performance sensitive code.
  */
 void kzfree(const void *p)
 {