selftests: filesystems: return Kselftest Skip code for skipped tests

When devpts_pts test is skipped because of unmet dependencies and/or
unsupported configuration, it exits with error which is treated as
a fail by the Kselftest framework. This leads to false negative
result even when the test could not be run.

In another case, it returns pass for a skipped test reporting a false
postive.

Change it to return kselftest skip code when a test gets skipped to
clearly report that the test could not be run.

Change it to use ksft_exit_skip() when test is skipped.

Signed-off-by: Shuah Khan (Samsung OSG) <[e-mail]>
Acked-by: Christian Brauner <[e-mail]>
Signed-off-by: Shuah Khan (Samsung OSG) <[e-mail]>

tools/testing/selftests/filesystems/devpts_pts.c

···
 #include <asm/ioctls.h>
 #include <sys/mount.h>
 #include <sys/wait.h>
+#include "../kselftest.h"
 
 static bool terminal_dup2(int duplicate, int original)
 {
···
 		if (errno == EINVAL) {
 			fprintf(stderr, "TIOCGPTPEER is not supported. "
 					"Skipping test.\n");
-			fret = EXIT_SUCCESS;
+			fret = KSFT_SKIP;
+		} else {
+			fprintf(stderr,
+				"Failed to perform TIOCGPTPEER ioctl\n");
+			fret = EXIT_FAILURE;
 		}
-
-		fprintf(stderr, "Failed to perform TIOCGPTPEER ioctl\n");
 		goto do_cleanup;
 	}
 
···
 	if (!isatty(STDIN_FILENO)) {
 		fprintf(stderr, "Standard input file descriptor is not attached "
 				"to a terminal. Skipping test\n");
-		exit(EXIT_FAILURE);
+		exit(KSFT_SKIP);
 	}
 
 	ret = unshare(CLONE_NEWNS);