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Problem:
On reconfigure of device, there is no way to defend if the backend
storage is matching with the initial backend storage.

Say, if an initial connect request for backend "pool1/image1" got
mapped to /dev/nbd0 and the userspace process is terminated. A next
reconfigure request within NBD_ATTR_DEAD_CONN_TIMEOUT is allowed to
use /dev/nbd0 for a different backend "pool1/image2"

For example, an operation like below could be dangerous:

$ sudo rbd-nbd map --try-netlink rbd-pool/ext4-image
/dev/nbd0
$ sudo blkid /dev/nbd0
/dev/nbd0: UUID="bfc444b4-64b1-418f-8b36-6e0d170cfc04" TYPE="ext4"
$ sudo pkill -9 rbd-nbd
$ sudo rbd-nbd attach --try-netlink --device /dev/nbd0 rbd-pool/xfs-image
/dev/nbd0
$ sudo blkid /dev/nbd0
/dev/nbd0: UUID="d29bf343-6570-4069-a9ea-2fa156ced908" TYPE="xfs"

Solution:
Provide a way for userspace processes to keep some metadata to identify
between the device and the backend, so that when a reconfigure request ...