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: 2b6d490b82668 ("selftests: drv-net: Factor out ksft C helpers")
Cc: Joe Damato <[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/lib/ksft.h

···
 		fd = STDOUT_FILENO;
 	}
 
-	write(fd, msg, sizeof(msg));
+	if (write(fd, msg, sizeof(msg)) < 0)
+		perror("write()");
 	if (fd != STDOUT_FILENO)
 		close(fd);
 }
···
 		fd = STDIN_FILENO;
 	}
 
-	read(fd, &byte, sizeof(byte));
+	if (read(fd, &byte, sizeof(byte)) < 0)
+		perror("read()");
 	if (fd != STDIN_FILENO)
 		close(fd);
 }