RDMA/rtrs: server: sysfs interface functions

This is the sysfs interface to rtrs sessions on server side:

  /sys/class/rtrs-server/<SESS-NAME>/
    *** rtrs session accepted from a client peer
    |
    |- paths/<SRC@DST>/
       *** established paths from a client in a session
       |
       |- disconnect
       |  *** disconnect path
       |
       |- hca_name
       |  *** HCA name
       |
       |- hca_port
       |  *** HCA port
       |
       |- stats/
          *** current path statistics
          |
	  |- rdma

Link: https://lore.kernel.org/r/20200511135131.27580-13-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>