compat: declare kmalloc_array() through LINUX_BACKPORT()

compat: declare kmalloc_array() through LINUX_BACKPORT()

This fixes compilation with Debian 3.2 kernels that
cherry picked the commit that added that call (a8203725).

commit a8203725dfded5c1f79dca3368a4a273e24b59bb
Author: Xi Wang <xi.wang@gmail.com>
Date:   Mon Mar 5 15:14:41 2012 -0800

    slab: introduce kmalloc_array()

    Introduce a kmalloc_array() wrapper that performs integer overflow
    checking without zeroing the memory.

    Suggested-by: Andrew Morton <akpm@linux-foundation.org>
    Suggested-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Xi Wang <xi.wang@gmail.com>
    Cc: Dan Carpenter <dan.carpenter@oracle.com>
    Acked-by: David Rientjes <rientjes@google.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>