RDMA/rtrs: server: statistics functions

This introduces set of functions used on server side to account statistics
of RDMA data sent/received.

Link: https://lore.kernel.org/r/20200511135131.27580-12-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>