nvme-multipath: drop head pointer check in nvme_mpath_clear_current_path()

A NS will always have a head pointer, so drop the check. As proof in
practice, all the nvme_mpath_clear_current_path() callers also
dereference ns->head.

This check has endured since the original changes to support multipath.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>

drivers/nvme/host/multipath.c

@@ -231,16 +231,12 @@ bool nvme_mpath_clear_current_path(struct nvme_ns *ns)
 	bool changed = false;
 	int node;
 
-	if (!head)
-		goto out;
-
 	for_each_node(node) {
 		if (ns == rcu_access_pointer(head->current_path[node])) {
 			rcu_assign_pointer(head->current_path[node], NULL);
 			changed = true;
 		}
 	}
-out:
 	return changed;
 }