zcache/debug: Use an array to initialize/use debugfs attributes.

It makes it neater and also allows us to piggyback on that
in the zcache_dump function.

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>