block/rnbd: server: main functionality

This is main functionality of rnbd-server module, which handles RTRS
events and rnbd protocol requests, like map (open) or unmap (close)
device.  Also server side is responsible for processing incoming IBTRS IO
requests and forward them to local mapped devices.

Link: https://lore.kernel.org/r/20200511135131.27580-21-danil.kipnis@cloud.ionos.comSigned-off-by: Danil Kipnis <danil.kipnis@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>