exit: Factor thread_group_exited out of pidfd_poll

Create an independent helper thread_group_exited which returns true
when all threads have passed exit_notify in do_exit.  AKA all of the
threads are at least zombies and might be dead or completely gone.

Create this helper by taking the logic out of pidfd_poll where it is
already tested, and adding a READ_ONCE on the read of
task->exit_state.

I will be changing the user mode driver code to use this same logic
to know when a user mode driver needs to be restarted.

Place the new helper thread_group_exited in kernel/exit.c and
EXPORT it so it can be used by modules.

Link: https://lkml.kernel.org/r/[email protected]
Acked-by: Christian Brauner <[email protected]>
Acked-by: Alexei Starovoitov <[email protected]>
Tested-by: Alexei Starovoitov <[email protected]>
Signed-off-by: "Eric W. Biederman" <[email protected]>

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