selftests: net: tfo: Fix build warning

Fix

tfo.c: In function ‘run_server’:
tfo.c:84:9: warning: ignoring return value of ‘read’ declared with attribute ‘warn_unused_result’

by evaluating the return value from read() and displaying an error message
if it reports an error.

Fixes: c65b5bb2329e3 ("selftests: net: add passive TFO test binary")
Cc: David Wei <[e-mail]>
Signed-off-by: Guenter Roeck <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/tfo.c

···
 	if (getsockopt(connfd, SOL_SOCKET, SO_INCOMING_NAPI_ID, &opt, &len) < 0)
 		error(1, errno, "getsockopt(SO_INCOMING_NAPI_ID)");
 
-	read(connfd, buf, 64);
+	if (read(connfd, buf, 64) < 0)
+		perror("read()");
 	fprintf(outfile, "%d\n", opt);
 
 	fclose(outfile);