when newidmap is not installed the code would hit the
reexec_in_user_namespace_wait code and wait for the child process to
be terminated. The child process is blocked waiting on the w pipe.
So make sure to unblock the child process first and then clean it up.
Closes: https://github.com/containers/podman/issues/7776
Signed-off-by: Giuseppe Scrivano <giuseppe@scrivano.org>