firmware: samsung: acpm: Fix false timeouts and Use-After-Free in polling

Sashiko identified severe races in the polling state machine [1].

In the ACPM driver's polling mode, threads waited for responses by
monitoring the globally shared 'bitmap_seqnum'. This caused false
timeouts because if a thread processed its response and freed the
sequence number, a concurrent TX thread could immediately reallocate
it before the polling thread woke up.

Additionally, the driver suffered from a cross-thread Use-After-Free
(UAF) preemption race. Previously, acpm_get_rx() cleared the sequence
number of whichever RX message it drained from the hardware queue. This
meant Thread A could globally free Thread B's sequence slot while
Thread B was asleep. A new Thread C could then steal the slot,
overwrite the buffer, and leave Thread B to wake up to corrupted state
or a timeout.

Fix this by rewriting the polling state machine:
1. Decouple polling from the global allocator by introducing a per-slot
   'completed' flag, synchronized via smp_store_release() and
   smp_load_acquire().
2. Strip acpm_get_saved_rx() out of acpm_get_rx() to make it a pure
   queue-draining function. Introduce a 'native_match' boolean argument
   which evaluates to true only if the thread natively processed its
   own sequence number during the call. This explicitly informs the
   polling loop whether it must retrieve its payload from the
   cross-thread cache.
3. Centralize the cache fallback and sequence number free (clear_bit)
   inside the polling loop. Crucially, the free operation now strictly
   targets the thread's own TX sequence number (xfer->txd[0]), rather
   than the drained RX sequence number. This enforces strict ownership:
   a thread only ever frees its own allocated sequence slot, and only
   at the exact moment it completes its poll, eliminating the UAF
   window.

Furthermore, explicitly guard the 'native_match' assignment with an
if (rx_seqnum == tx_seqnum) check, even for zero-length (no payload)
responses. While an unguarded assignment wouldn't crash (because the
cache fallback acpm_get_saved_rx() safely returns early on zero-length
transfers) doing so would "lie" to the state machine. If a thread
drained the queue and found another thread's zero-length message,
setting native_match = true would falsely convince the polling loop
that it natively handled its own response. Maintaining a rigorous state
machine requires that native_match is only set when a thread explicitly
processes its own sequence number.

Cc: stable@vger.kernel.org
Fixes: a88927b534ba ("firmware: add Exynos ACPM protocol driver")
Closes: https://sashiko.dev/#/patchset/20260429-acpm-fixes-sashiko-reports-v3-0-47cf74ab09ad%40linaro.org [1]
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Link: https://patch.msgid.link/20260505-acpm-fixes-sashiko-reports-v5-5-43b5ee7f1674@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>

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