block/rnbd: client: sysfs interface functions

This is the sysfs interface to rnbd block devices on client side:

  /sys/class/rnbd-client/ctl/
    |- map_device
    |  *** maps remote device
    |
    |- devices/
       *** all mapped devices

  /sys/block/rnbd<N>/rnbd/
    |- unmap_device
    |  *** unmaps device
    |
    |- state
    |  *** device state
    |
    |- session
    |  *** session name
    |
    |- mapping_path
       *** path of the dev that was mapped on server

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Danil Kipnis <[email protected]>
Signed-off-by: Jack Wang <[email protected]>
Reviewed-by: Bart Van Assche <[email protected]>
Acked-by: Jens Axboe <[email protected]>
Signed-off-by: Jason Gunthorpe <[email protected]>

Author: Jack Wang