RDMA/rtrs: client: statistics functions

This introduces set of functions used on client side to account statistics
of RDMA data sent/received, amount of IOs inflight, latency, cpu
migrations, etc.  Almost all statistics are collected using percpu
variables.

Link: https://lore.kernel.org/r/20200511135131.27580-8-danil.kipnis@cloud.ionos.com


Signed-off-by: Danil Kipnis <danil.kipnis@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>