selftests/pidfd: add CLONE_AUTOREAP tests

Add tests for the new CLONE_AUTOREAP clone3() flag:

- autoreap_without_pidfd: CLONE_AUTOREAP without CLONE_PIDFD works
  (fire-and-forget)
- autoreap_rejects_exit_signal: CLONE_AUTOREAP with non-zero
  exit_signal fails
- autoreap_rejects_parent: CLONE_AUTOREAP with CLONE_PARENT fails
- autoreap_rejects_thread: CLONE_AUTOREAP with CLONE_THREAD fails
- autoreap_basic: child exits, pidfd poll works, PIDFD_GET_INFO returns
  correct exit code, waitpid() returns -ECHILD
- autoreap_signaled: child killed by signal, exit info correct via pidfd
- autoreap_reparent: autoreap grandchild reparented to subreaper still
  auto-reaps
- autoreap_multithreaded: autoreap process with sub-threads auto-reaps
  after last thread exits
- autoreap_no_inherit: grandchild forked without CLONE_AUTOREAP becomes
  a regular zombie

Link: https://patch.msgid.link/20260226-work-pidfs-autoreap-v5-4-d148b984a989@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

tools/testing/selftests/pidfd/.gitignore

@@ -12,3 +12,4 @@ pidfd_info_test
 pidfd_exec_helper
 pidfd_xattr_test
 pidfd_setattr_test
+pidfd_autoreap_test

tools/testing/selftests/pidfd/Makefile

@@ -4,7 +4,7 @@ CFLAGS += -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES) -pthread -Wall
 TEST_GEN_PROGS := pidfd_test pidfd_fdinfo_test pidfd_open_test \
 	pidfd_poll_test pidfd_wait pidfd_getfd_test pidfd_setns_test \
 	pidfd_file_handle_test pidfd_bind_mount pidfd_info_test \
-	pidfd_xattr_test pidfd_setattr_test
+	pidfd_xattr_test pidfd_setattr_test pidfd_autoreap_test
 
 TEST_GEN_PROGS_EXTENDED := pidfd_exec_helper