mm/kfence: randomize the freelist on initialization

mm/kfence: randomize the freelist on initialization

Randomize the KFENCE freelist during pool initialization to make
allocation patterns less predictable.  This is achieved by shuffling the
order in which metadata objects are added to the freelist using
get_random_u32_below().

Additionally, ensure the error path correctly calculates the address range
to be reset if initialization fails, as the address increment logic has
been moved to a separate loop.

Link: https://lkml.kernel.org/r/20260120161510.3289089-1-pimyn@google.com
Fixes: 0ce20dd84089 ("mm: add Kernel Electric-Fence infrastructure")
Signed-off-by: Pimyn Girgis <pimyn@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Ernesto Martnez Garca <ernesto.martinezgarcia@tugraz.at>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Kees Cook <kees@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>