mptcp: add support for TCP_FASTOPEN_KEY sockopt

The goal of this socket option is to set different keys per listener,
see commit 1fba70e5 ("tcp: socket option to set TCP fast open key")
for more details about this socket option.

The only thing to do here with MPTCP is to relay the request to the
first subflow like it is already done for the other TCP_FASTOPEN* socket
options.
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/sockopt.c

@@ -561,6 +561,7 @@ static bool mptcp_supported_sockopt(int level, int optname)
 		case TCP_INQ:
 		case TCP_FASTOPEN:
 		case TCP_FASTOPEN_CONNECT:
+		case TCP_FASTOPEN_KEY:
 		case TCP_FASTOPEN_NO_COOKIE:
 			return true;
 		}
@@ -570,9 +571,6 @@ static bool mptcp_supported_sockopt(int level, int optname)
 		/* TCP_REPAIR, TCP_REPAIR_QUEUE, TCP_QUEUE_SEQ, TCP_REPAIR_OPTIONS,
 		 * TCP_REPAIR_WINDOW are not supported, better avoid this mess
 		 */
-		/* TCP_FASTOPEN_KEY is not supported because
-		 * fastopen for the listener side is currently unsupported
-		 */
 	}
 	return false;
 }
@@ -804,6 +802,7 @@ static int mptcp_setsockopt_sol_tcp(struct mptcp_sock *msk, int optname,
 		return 0;
 	case TCP_FASTOPEN:
 	case TCP_FASTOPEN_CONNECT:
+	case TCP_FASTOPEN_KEY:
 	case TCP_FASTOPEN_NO_COOKIE:
 		return mptcp_setsockopt_first_sf_only(msk, SOL_TCP, optname,
 						      optval, optlen);
@@ -1170,6 +1169,7 @@ static int mptcp_getsockopt_sol_tcp(struct mptcp_sock *msk, int optname,
 	case TCP_DEFER_ACCEPT:
 	case TCP_FASTOPEN:
 	case TCP_FASTOPEN_CONNECT:
+	case TCP_FASTOPEN_KEY:
 	case TCP_FASTOPEN_NO_COOKIE:
 		return mptcp_getsockopt_first_sf_only(msk, SOL_TCP, optname,
 						      optval, optlen);