Grow: store a link to current backup file in /run/mdadm or similar.
Subsequent patch will allow the background part of "mdadm --grow" to
be run from systemd. This can require the passing of a backup file
name.
To do this, store that name as a symlink in /run/mdadm (or MAP_DIR)
and look for it when appropriate.
It might be useful to also store the name across reboot, but that
would be a different patch. We would need to use the uuid to identify
it, and store it in stable storage.