selftests: net: tfo: Fix build warning

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 <dw@davidwei.uk>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Link: https://patch.msgid.link/20251205171010.515236-14-linux@roeck-us.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>