Support adding a spare to a live md array with external metadata.

i.e. extend the 'md/dev-XXX/slot' attribute so that you can
tell a device to fill an vacant slot in an and md array.

Signed-off-by: Neil Brown <neilb@suse.de>
diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index a71277b..50ad8d2 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -1116,6 +1116,8 @@
 	int found = 0;
 	int mirror;
 	mirror_info_t *p;
+	int first = 0;
+	int last = mddev->raid_disks - 1;
 
 	if (mddev->recovery_cp < MaxSector)
 		/* only hot-add to in-sync arrays, as recovery is
@@ -1125,12 +1127,16 @@
 	if (!enough(conf))
 		return 0;
 
+	if (rdev->raid_disk)
+		first = last = rdev->raid_disk;
+
 	if (rdev->saved_raid_disk >= 0 &&
+	    rdev->saved_raid_disk >= first &&
 	    conf->mirrors[rdev->saved_raid_disk].rdev == NULL)
 		mirror = rdev->saved_raid_disk;
 	else
-		mirror = 0;
-	for ( ; mirror < mddev->raid_disks; mirror++)
+		mirror = first;
+	for ( ; mirror <= last ; mirror++)
 		if ( !(p=conf->mirrors+mirror)->rdev) {
 
 			blk_queue_stack_limits(mddev->queue,