RDMA/rtrs-clt: New sysfs attribute to print the latency of each path

It shows the latest latency that the client checked when sending the
heart-beat.

Link: https://lore.kernel.org/r/20210407113444.150961-3-gi-oh.kim@ionos.com


Signed-off-by: Gioh Kim <gi-oh.kim@ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>