selftests: net: Fix build warnings

Fix

ksft.h: In function ‘ksft_ready’:
ksft.h:27:9: warning: ignoring return value of ‘write’ declared with attribute ‘warn_unused_result’

ksft.h: In function ‘ksft_wait’:
ksft.h:51:9: warning: ignoring return value of ‘read’ declared with attribute ‘warn_unused_result’

by checking the return value of the affected functions and displaying
an error message if an error is seen.

Fixes: 2b6d490 ("selftests: drv-net: Factor out ksft C helpers")
Cc: Joe Damato <jdamato@fastly.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Link: https://patch.msgid.link/20251205171010.515236-11-linux@roeck-us.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: groeck

tools/testing/selftests/net/lib/ksft.h

@@ -24,7 +24,8 @@ static inline void ksft_ready(void)
 		fd = STDOUT_FILENO;
 	}
 
-	write(fd, msg, sizeof(msg));
+	if (write(fd, msg, sizeof(msg)) < 0)
+		perror("write()");
 	if (fd != STDOUT_FILENO)
 		close(fd);
 }
@@ -48,7 +49,8 @@ static inline void ksft_wait(void)
 		fd = STDIN_FILENO;
 	}
 
-	read(fd, &byte, sizeof(byte));
+	if (read(fd, &byte, sizeof(byte)) < 0)
+		perror("read()");
 	if (fd != STDIN_FILENO)
 		close(fd);
 }