net/netlink: Add getsockopt support for NETLINK_LISTEN_ALL_NSID

Currently getsockopt does not support NETLINK_LISTEN_ALL_NSID,
and we are unable to get the value of NETLINK_LISTEN_ALL_NSID
socket option through getsockopt.

This patch adds getsockopt support for NETLINK_LISTEN_ALL_NSID.
Signed-off-by: Juntong Deng <juntong.deng@outlook.com>
Link: https://lore.kernel.org/r/AM6PR03MB58482322B7B335308DA56FE599272@AM6PR03MB5848.eurprd03.prod.outlook.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/netlink/af_netlink.c

@@ -1773,6 +1773,9 @@ static int netlink_getsockopt(struct socket *sock, int level, int optname,
 		netlink_unlock_table();
 		return err;
 	}
+	case NETLINK_LISTEN_ALL_NSID:
+		flag = NETLINK_F_LISTEN_ALL_NSID;
+		break;
 	case NETLINK_CAP_ACK:
 		flag = NETLINK_F_CAP_ACK;
 		break;