zcache/debug: Coalesce all debug under CONFIG_ZCACHE_DEBUG

and also define this extra attribute in the Kconfig entry.

Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>