NVMe-OF is configured using configfs. The target is specified by the
user writing a path to a configfs attribute. This is the way it works
today but with blkdev_get_by_path()[1]. For the passthru code, we need
to get a nvme_ctrl instead of a block_device, but the principal is the same.
Why isn't a fd being passed in there instead of a random string?
I suppose we could echo a string of the file descriptor number there,
and look up the fd in the process' file descriptor table ...
I'll get my coat.