md: Don't allow slot_store while resync/recovery is happening.

Activating a spare in an array while resync/recovery is already
happening can lead the that spare being marked in-sync when it isn't
really.
So don't allow the 'slot' to be set (this activating the device)
while resync/recovery is happening.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -2463,6 +2463,9 @@ slot_store(mdk_rdev_t *rdev, const char *buf, size_t len)
 		if (rdev->raid_disk != -1)
 			return -EBUSY;
 
+		if (test_bit(MD_RECOVERY_RUNNING, &rdev->mddev->recovery))
+			return -EBUSY;
+
 		if (rdev->mddev->pers->hot_add_disk == NULL)
 			return -EINVAL;