srcutiny: Add an atomic Tiny SRCU

This commit adds the Tiny SRCU counterpart to Tree SRCU's
synchronize_srcu_atomic().  One might hope that this could be as trivial
as Tiny RCU's synchronize_rcu(), and there was a time when it would
have been.  But lazy preemption really can preempt an SRCU read-side
critical section, which means that synchronize_srcu_atomic() really must
be prepared to spin waiting for it.

This spinning currently consists of cond_resched_tasks_rcu_qs()
and cpu_relax().  It would be better to have some way of telling the
scheduler that there is nothing useful for us to do.  We cannot use
the traditional wait_event() approach because synchronize_srcu_atomic()
is not permitted to block.

[ paulmck: Apply Kunwu Chan and Joel Fernandes feedback. ]

Co-developed-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: Claude:claude-mythos-5
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

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