zram: destroy all devices on error recovery path in zram_init()

Date	Mon, 3 Jun 2013 23:42:16 +0800

On error recovery path of zram_init(), it leaks the zram device object
causing the failure.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Paul Reioux <reioux@gmail.com>