nvme: fix srcu locking on error return in nvme_get_ns_from_disk

If we can't get a namespace don't leak the SRCU lock.  nvme_ioctl was
working around this, but nvme_pr_command wasn't handling this properly.
Just do what callers would usually expect.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>

drivers/nvme/host/core.c

@@ -1361,9 +1361,14 @@ static struct nvme_ns *nvme_get_ns_from_disk(struct gendisk *disk,
 {
 #ifdef CONFIG_NVME_MULTIPATH
 	if (disk->fops == &nvme_ns_head_ops) {
+		struct nvme_ns *ns;
+
 		*head = disk->private_data;
 		*srcu_idx = srcu_read_lock(&(*head)->srcu);
-		return nvme_find_path(*head);
+		ns = nvme_find_path(*head);
+		if (!ns)
+			srcu_read_unlock(&(*head)->srcu, *srcu_idx);
+		return ns;
 	}
 #endif
 	*head = NULL;
@@ -1410,9 +1415,9 @@ static int nvme_ioctl(struct block_device *bdev, fmode_t mode,
 
 	ns = nvme_get_ns_from_disk(bdev->bd_disk, &head, &srcu_idx);
 	if (unlikely(!ns))
-		ret = -EWOULDBLOCK;
-	else
-		ret = nvme_ns_ioctl(ns, cmd, arg);
+		return -EWOULDBLOCK;
+
+	ret = nvme_ns_ioctl(ns, cmd, arg);
 	nvme_put_ns_from_disk(head, srcu_idx);
 	return ret;
 }