nbd: provide a way for userspace processes to identify device backends

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 is
made, we can compare and avoid such dangerous operations.

With this solution, as part of the initial connect request, backend
path can be stored in the sysfs per device config, so that on a reconfigure
request it's easy to check if the backend path matches with the initial
connect backend path.

Please note, ioctl interface to nbd will not have these changes, as there
won't be any reconfigure.

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20210429102828.31248-1-prasanna.kalever@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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