hung_task: enable runtime reset of hung_task_detect_count

Currently, the hung_task_detect_count sysctl provides a cumulative count
of hung tasks since boot. In long-running, high-availability
environments, this counter may lose its utility if it cannot be reset once
an incident has been resolved. Furthermore, the previous implementation
relied upon implicit ordering, which could not strictly guarantee that
diagnostic metadata published by one CPU was visible to the panic logic on
another.

This patch introduces the capability to reset the detection count by
writing "0" to the hung_task_detect_count sysctl. The proc_handler logic
has been updated to validate this input and atomically reset the counter.

The synchronisation of sysctl_hung_task_detect_count relies upon a
transactional model to ensure the integrity of the detection counter
against concurrent resets from userspace. The application of
atomic_long_read_acquire() and atomic_long_cmpxchg_release() is correct
and provides the following guarantees:

1. Prevention of Load-Store Reordering via Acquire Semantics By
utilising atomic_long_read_acquire() to snapshot the counter
before initiating the task traversal, we establish a strict
memory barrier. This prevents the compiler or hardware from
reordering the initial load to a point later in the scan. Without
this "acquire" barrier, a delayed load could potentially read a
"0" value resulting from a userspace reset that occurred
mid-scan. This would lead to the subsequent cmpxchg succeeding
erroneously, thereby overwriting the user's reset with stale
increment data.

2. Atomicity of the "Commit" Phase via Release Semantics The
atomic_long_cmpxchg_release() serves as the transaction's commit
point. The "release" barrier ensures that all diagnostic
recordings and task-state observations made during the scan are
globally visible before the counter is incremented.

3. Race Condition Resolution This pairing effectively detects any
"out-of-band" reset of the counter. If
sysctl_hung_task_detect_count is modified via the procfs
interface during the scan, the final cmpxchg will detect the
discrepancy between the current value and the "acquire" snapshot.
Consequently, the update will fail, ensuring that a reset command
from the administrator is prioritised over a scan that may have
been invalidated by that very reset.

Link: https://lkml.kernel.org/r/20260303203031.4097316-3-atomlin@atomlin.com
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Joel Granados <joel.granados@kernel.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Lance Yang <lance.yang@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>