zram/xvmalloc: create CONFIG_ZRAM_DEBUG for debug code

Add a debug config flag to enable debug printk output and future
debug code.

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>