RCU pull request for v6.18

This pull request contains the following branches, non-octopus merged:

Documentation updates:

  - Update whatisRCU.rst and checklist.rst for recent RCU API additions.

  - Fix RCU documentation formatting and typos.

  - Replace dead Ottawa Linux Symposium links in RTFP.txt.

Miscellaneous RCU updates:

  - Document that rcu_barrier() hurries RCU_LAZY callbacks.

  - Remove redundant interrupt disabling from
    rcu_preempt_deferred_qs_handler().

  - Move list_for_each_rcu from list.h to rculist.h, and adjust the
    include directive in kernel/cgroup/dmem.c accordingly.

  - Make initial set of changes to accommodate upcoming system_percpu_wq
    changes.

SRCU updates:

  - Create an srcu_read_lock_fast_notrace() for eventual use in tracing,
    including adding guards.

  - Document the reliance on per-CPU operations as implicit RCU readers
    in __srcu_read_{,un}lock_fast().

  - Document the srcu_flip() function's memory-barrier D's relationship
    to SRCU-fast readers.

  - Remove a redundant preempt_disable() and preempt_enable() pair from
    srcu_gp_start_if_needed().

Torture-test updates:

  - Fix jitter.sh spin time so that it actually varies as advertised.
    It is still quite coarse-grained, but at least it does now vary.

  - Update torture.sh help text to include the not-so-new --do-normal
    parameter, which permits (for example) testing KCSAN kernels without
    doing non-debug kernels.

  - Fix a number of false-positive diagnostics that were being triggered
    by rcutorture starting before boot completed.  Running multiple
    near-CPU-bound rcutorture processes when there is only the boot CPU
    is after all a bit excessive.

  - Substitute kcalloc() for kzalloc().

  - Remove a redundant kfree() and NULL out kfree()ed objects.

Merge branch 'torture.2025.08.14a' into HEAD

Torture-test updates:
* rcutorture: Fix jitter.sh spin time
* torture: Add --do-normal parameter to torture.sh help text
* torture: Announce kernel boot status at torture-test startup
* rcutorture: Suppress "Writer stall state" reports during boot
* rcutorture: Delay rcutorture readers and writers until boot completes
* torture: Delay CPU-hotplug operations until boot completes
* rcutorture: Delay forward-progress testing until boot completes
* rcutorture,refscale: Use kcalloc() instead of kzalloc()
* refperf: Remove redundant kfree() after torture_stop_kthread()
* refperf: Set reader_tasks to NULL after kfree()