staging: zcache: introduce zero-filled page stat count

Introduce zero-filled page statistics to monitor the number of
zero-filled pages.

Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --cc drivers/staging/zcache/debug.c
Simple merge

diff --cc drivers/staging/zcache/debug.h
Simple merge

diff --cc drivers/staging/zcache/zcache-main.c
Simple merge