staging: zcache: fix account foregin counters against zero-filled pages

zero-filled pages won't be compressed and sent to remote system. Monitor
the number ephemeral and persistent pages that Ramster has sent make no
sense. This patch skip account foregin counters against zero-filled pages.

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