nbd: null check for nla_nest_start

nla_nest_start() may fail and return NULL. Insert a check and set errno
based on other call sites within the same source code.

Signed-off-by: Navid Emamdoost <[e-mail]>
Reviewed-by: Michal Kubecek <[e-mail]>
Fixes: 47d902b90a32 ("nbd: add a status netlink command")
Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/nbd.c

···
 	}
 
 	dev_list = nla_nest_start_noflag(reply, NBD_ATTR_DEVICE_LIST);
+	if (!dev_list) {
+		nlmsg_free(reply);
+		ret = -EMSGSIZE;
+		goto out;
+	}
+
 	if (index == -1) {
 		ret = idr_for_each(&nbd_index_idr, &status_cb, reply);
 		if (ret) {