selftests: fixup build warnings in pidfd / clone3 tests

These are some trivial fixups, which were needed to build the tests with
clang and -Werror. The following issues are fixed:

- Remove various unused variables.
- In child_poll_leader_exit_test, clang isn't smart enough to realize
  syscall(SYS_exit, 0) won't return, so it complains we never return
  from a non-void function. Add an extra exit(0) to appease it.
- In test_pidfd_poll_leader_exit, ret may be branched on despite being
  uninitialized, if we have !use_waitpid. Initialize it to zero to get
  the right behavior in that case.

Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/clone3/clone3.c

@@ -126,8 +126,6 @@ static void test_clone3(uint64_t flags, size_t size, int expected,
 
 int main(int argc, char *argv[])
 {
-	pid_t pid;
-
 	uid_t uid = getuid();
 
 	ksft_print_header();

tools/testing/selftests/pidfd/pidfd_test.c

@@ -441,7 +441,6 @@ static void test_pidfd_poll_exec(int use_waitpid)
 {
 	int pid, pidfd = 0;
 	int status, ret;
-	pthread_t t1;
 	time_t prog_start = time(NULL);
 	const char *test_name = "pidfd_poll check for premature notification on child thread exec";
 
@@ -500,13 +499,14 @@ static int child_poll_leader_exit_test(void *args)
 	 */
 	*child_exit_secs = time(NULL);
 	syscall(SYS_exit, 0);
+	/* Never reached, but appeases compiler thinking we should return. */
+	exit(0);
 }
 
 static void test_pidfd_poll_leader_exit(int use_waitpid)
 {
 	int pid, pidfd = 0;
-	int status, ret;
-	time_t prog_start = time(NULL);
+	int status, ret = 0;
 	const char *test_name = "pidfd_poll check for premature notification on non-empty"
 				"group leader exit";
 

tools/testing/selftests/pidfd/pidfd_wait.c

@@ -39,15 +39,14 @@ static int sys_waitid(int which, pid_t pid, siginfo_t *info, int options,
 
 TEST(wait_simple)
 {
-	int pidfd = -1, status = 0;
+	int pidfd = -1;
 	pid_t parent_tid = -1;
 	struct clone_args args = {
 		.parent_tid = ptr_to_u64(&parent_tid),
 		.pidfd = ptr_to_u64(&pidfd),
 		.flags = CLONE_PIDFD | CLONE_PARENT_SETTID,
 		.exit_signal = SIGCHLD,
 	};
-	int ret;
 	pid_t pid;
 	siginfo_t info = {
 		.si_signo = 0,
@@ -88,7 +87,7 @@ TEST(wait_simple)
 
 TEST(wait_states)
 {
-	int pidfd = -1, status = 0;
+	int pidfd = -1;
 	pid_t parent_tid = -1;
 	struct clone_args args = {
 		.parent_tid = ptr_to_u64(&parent_tid),