srcutree: Add an atomic Tree SRCU

Some dedicated srcu_struct users have read-side critical sections
which are short, never sleep, and never block on anything which may
itself depend on memory allocation — because they were, until
recently, spinlock or rwlock critical sections. For such a domain the
update side can safely wait for readers by spinning, from contexts
where sleeping is undesirable or the grace-period machinery's
latency (workqueue scheduling, jiffy-paced retries) dominates the
actual reader drain time.

But that is only safe if every reader keeps the promise. Make the
promise explicit and machine-checkable:

 - srcu_read_lock_atomic() / srcu_read_unlock_atomic() enter the
   usual (smp_mb-based) read-side critical section with preemption
   disabled and (except in hardirq, where it is redundant)
   non_block_start() armed, recording SRCU_READ_FLAVOR_ATOMIC in the
   per-CPU reader flavor. Disabling preemption enforces the
   no-sleeping promise on every configuration and bounds the section,
   so it is always running on some CPU; non_block_start() extends the
   enforcement to even potentially-sleeping calls on paths which
   happen not to block. The existing reader-flavor consistency checks
   complain about any mixing with other flavors.

 - synchronize_srcu_atomic() waits for all pre-existing readers by
   repeating the try_synchronize_srcu() both-epoch counter proof with
   cpu_relax() until it succeeds: no sleeping, no index flip, no
   grace-period sequence update, and therefore no interaction with
   concurrent call_srcu(), synchronize_srcu() or srcu_barrier(). It
   always provides the full grace-period guarantee: if the domain
   turns out to have had readers of any other flavor — a caller bug,
   since such a reader may be asleep and spinning on it would be
   unbounded — it complains and falls back to a real (sleeping) grace
   period internally, that being the only correct wait for a
   possibly-sleeping reader. The flavor mask is rechecked on every
   iteration so a first non-atomic reader appearing mid-spin takes
   the same path.

The immediate motivation is the proposed conversion of KVM's
gfn_to_pfn_cache to SRCU¹, whose mmu_notifier invalidation path drains
readers before the primary MMU zaps a page. With the readers declared
atomic, that drain becomes spin-only: no sleeping at all in the
notifier, bounded by the longest reader section, satisfying even the
strictest reading of the OOM-reaper non-blocking requirement without
needing to touch the non_block_start() annotation².

This commit also adds atomic-SRCU-specific initializers:
init_srcu_struct_atomic(), DEFINE_SRCU_ATOMIC(), and
DEFINE_STATIC_SRCU_ATOMIC().

This commit implements only Tree SRCU.  Tiny SRCU will follow.

¹ https://lore.kernel.org/all/20260811132237.102400-1-dwmw2@infradead.org/
² https://lore.kernel.org/all/20260812134934.GC662699@ziepe.ca/

[ paulmck: Apply Kunwu Chan 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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