stdev is computed in `cephfs-top` tool - clients forward
square of sums and IO count required to calculate stdev.
Signed-off-by: NVenky Shankar <vshankar@redhat.com>
Reviewed-by: NXiubo Li <xiubli@redhat.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>