staging: zcache: fix pers_pageframes|_max aren't exported in debugfs

Before commit 9c0ad59e ("zcache/debug: Use an array to initialize/use
debugfs attributes"), pers_pageframes|_max are exported in debugfs, but
this commit forgot use array export pers_pageframes|_max. This patch add
pers_pageframes|_max back.

Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>