rnbd-srv: replace sess->open_flags with a "bool readonly"

Stop passing the fmode_t around and just use a simple bool to track if
an export is read-only.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Acked-by: Christian Brauner <brauner@kernel.org>
Acked-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20230608110258.189493-24-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>