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>