ceph: track average r/w/m latency

Make the math a bit simpler to understand (should not
affect execution speeds).

Signed-off-by: Venky Shankar <vshankar@redhat.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>