nvme: fix RCU hole that allowed for endless looping in multipath round robin

Make nvme_ns_remove match the assumptions elsewhere.

1) !NVME_NS_READY needs to be srcu synchronized to make sure nothing is
   running in __nvme_find_path or nvme_round_robin_path that will
   re-assign this ns to current_path.

2) Any matching current_path entries need to be cleared before removing
   from the siblings list, to prevent calling nvme_round_robin_path with
   an "old" ns that's off list.

3) Finally the list_del_rcu can happen, and then synchronize again
   before releasing any reference counts.
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/core.c

@@ -4031,6 +4031,16 @@ static void nvme_ns_remove(struct nvme_ns *ns)
 	set_capacity(ns->disk, 0);
 	nvme_fault_inject_fini(&ns->fault_inject);
 
+	/*
+	 * Ensure that !NVME_NS_READY is seen by other threads to prevent
+	 * this ns going back into current_path.
+	 */
+	synchronize_srcu(&ns->head->srcu);
+
+	/* wait for concurrent submissions */
+	if (nvme_mpath_clear_current_path(ns))
+		synchronize_srcu(&ns->head->srcu);
+
 	mutex_lock(&ns->ctrl->subsys->lock);
 	list_del_rcu(&ns->siblings);
 	if (list_empty(&ns->head->list)) {
@@ -4042,10 +4052,6 @@ static void nvme_ns_remove(struct nvme_ns *ns)
 	/* guarantee not available in head->list */
 	synchronize_rcu();
 
-	/* wait for concurrent submissions */
-	if (nvme_mpath_clear_current_path(ns))
-		synchronize_srcu(&ns->head->srcu);
-
 	if (!nvme_ns_head_multipath(ns->head))
 		nvme_cdev_del(&ns->cdev, &ns->cdev_device);
 	del_gendisk(ns->disk);