block/rnbd-proto: Check and retain the NOUNMAP flag for requests

block/rnbd-proto: Check and retain the NOUNMAP flag for requests

The NOUNMAP flag is in combination with WRITE_ZEROES flag to indicate
that the upper layers wants the sectors zeroed, but does not want it to
get freed. This instruction is especially important for storage stacks
which involves a layer capable of thin provisioning.

This commit makes RNBD block device transfer and retain this NOUNMAP flag
for requests, so it can be passed onto the backend device on the server
side.

Since it is a change in the wire protocol, bump the minor version of
protocol.

Signed-off-by: Md Haris Iqbal <haris.iqbal@ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Grzegorz Prajsner <grzegorz.prajsner@ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>