nvme: fix a crash in nvme_mpath_add_disk

For private namespaces ns->head_disk is NULL, so add a NULL check
before updating the BDI capabilities.

Fixes: b2ce4d90690b ("nvme-multipath: set bdi capabilities once")
Reported-by: Avinash M N <[e-mail]>
Signed-off-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Sagi Grimberg <[e-mail]>
Reviewed-by: Max Gurtovoy <[e-mail]>

drivers/nvme/host/multipath.c

···
 	}
 
 	if (bdi_cap_stable_pages_required(ns->queue->backing_dev_info)) {
-		struct backing_dev_info *info =
-					ns->head->disk->queue->backing_dev_info;
+		struct gendisk *disk = ns->head->disk;
 
-		info->capabilities |= BDI_CAP_STABLE_WRITES;
+		if (disk)
+			disk->queue->backing_dev_info->capabilities |=
+					BDI_CAP_STABLE_WRITES;
 	}
 }