staging: zram: handle mem suffixes in disk size zram_sysfs parameter

Use memparse() to allow mem suffixes in disksize sysfs number.
Examples:
    echo 256K > /sys/block/zram0/disksize
    echo 512M > /sys/block/zram0/disksize
    echo 1G > /sys/block/zram0/disksize

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>