selftests: pidfd: Fix compling warnings

Fix warnings and enable Wall.

pidfd_wait.c: In function ‘wait_nonblock’:
pidfd_wait.c:150:13: warning: unused variable ‘status’ [-Wunused-variable]
  150 |  int pidfd, status = 0;
      |             ^~~~~~
...
pidfd_test.c: In function ‘child_poll_exec_test’:
pidfd_test.c:438:1: warning: no return statement in function returning non-void [-Wreturn-type]
  438 | }
      | ^
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
v2: fix mistake assignment to pidfd
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/pidfd/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
-CFLAGS += -g -I../../../../usr/include/ -pthread
+CFLAGS += -g -I../../../../usr/include/ -pthread -Wall
 
 TEST_GEN_PROGS := pidfd_test pidfd_fdinfo_test pidfd_open_test \
 	pidfd_poll_test pidfd_wait pidfd_getfd_test pidfd_setns_test

tools/testing/selftests/pidfd/pidfd_test.c

@@ -435,6 +435,8 @@ static int child_poll_exec_test(void *args)
 	 */
 	while (1)
 		sleep(1);
+
+	return 0;
 }
 
 static void test_pidfd_poll_exec(int use_waitpid)

tools/testing/selftests/pidfd/pidfd_wait.c

@@ -148,7 +148,7 @@ TEST(wait_states)
 
 TEST(wait_nonblock)
 {
-	int pidfd, status = 0;
+	int pidfd;
 	unsigned int flags = 0;
 	pid_t parent_tid = -1;
 	struct clone_args args = {