zram/xvmalloc: Close 32byte hole on 64bit CPUs

By swapping the total_pages statistic with the lock we close a
hole in the structure for 64-bit CPUs.

Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>