block/rnbd: client: main functionality

This is main functionality of rnbd-client module, which provides interface
to map remote device as local block device /dev/rnbd<N> and feeds RTRS
with IO requests.

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]>