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: NDanil Kipnis <danil.kipnis@cloud.ionos.com>
Signed-off-by: NJack Wang <jinpu.wang@cloud.ionos.com>
Reviewed-by: NBart Van Assche <bvanassche@acm.org>
Acked-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>