selftests/bpf: Use start_server_str in mptcp

Since start_server_str() is added now, it can be used in mptcp.c in
start_mptcp_server() instead of using helpers make_sockaddr() and
start_server_addr() to simplify the code.
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/16fb3e2cd60b64b5470b0e69f1aa233feaf2717c.1718932493.git.tanggeliang@kylinos.cnSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/mptcp.c

@@ -89,13 +89,8 @@ static int start_mptcp_server(int family, const char *addr_str, __u16 port,
 		.timeout_ms	= timeout_ms,
 		.proto		= IPPROTO_MPTCP,
 	};
-	struct sockaddr_storage addr;
-	socklen_t addrlen;
 
-	if (make_sockaddr(family, addr_str, port, &addr, &addrlen))
-		return -1;
-
-	return start_server_addr(SOCK_STREAM, &addr, addrlen, &opts);
+	return start_server_str(family, SOCK_STREAM, addr_str, port, &opts);
 }
 
 static int verify_tsk(int map_fd, int client_fd)