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.

Signed-off-by: NeilBrown <neilb@suse.de>
Grow: try to let "--grow --continue" from systemd complete a reshape.

If "--assemble" or "--incremental" is started by udev, then
monitoring the reshape in the background won't work.

So try asking systemd to start a grow-continue.

If that fails, just do it the old way.

Signed-off-by: NeilBrown <neilb@suse.de>