RDMA/rtrs: client: sysfs interface functions

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

  /sys/class/rtrs-client/<SESS-NAME>/
    *** rtrs session created by rtrs_clt_open() API call
    |
    |- max_reconnect_attempts
    |  *** number of reconnect attempts for session
    |
    |- add_path
    |  *** adds another connection path into rtrs session
    |
    |- paths/<SRC@DST>/
       *** established paths to server in a session
       |
       |- disconnect
       |  *** disconnect path
       |
       |- reconnect
       |  *** reconnect path
       |
       |- remove_path
       |  *** remove current path
       |
       |- state
       |  *** retrieve current path state
       |
       |- hca_port
       |  *** HCA port number
       |
       |- hca_name
       |  *** HCA name
       |
       |- stats/
          *** current path statistics
          |
	  |- cpu_migration
	  |- rdma
	  |- reconnects
	  |- reset_all

Link: https://lore.kernel.org/r/20200511135131.27580-9-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>