md/linear: fix hot-add of devices to linear arrays.

commit d70ed2e4fafdbef0800e73942482bb075c21578b
broke hot-add to a linear array.
After that commit, metadata if not written to devices until they
have been fully integrated into the array as determined by
saved_raid_disk.  That patch arranged to clear that field after
a recovery completed.

However for linear arrays, there is no recovery - the integration is
instantaneous.  So we need to explicitly clear the saved_raid_disk
field.

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

drivers/md/linear.c

@@ -230,6 +230,7 @@ static int linear_add(struct mddev *mddev, struct md_rdev *rdev)
 		return -EINVAL;
 
 	rdev->raid_disk = rdev->saved_raid_disk;
+	rdev->saved_raid_disk = -1;
 
 	newconf = linear_conf(mddev,mddev->raid_disks+1);