workqueue: Changes for v6.18

- WQ_PERCPU was added to remaining alloc_workqueue() users and system_wq
  usage was replaced with system_percpu_wq and system_unbound_wq with
  system_dfl_wq. These are equivalent conversions with no functional changes,
  preparing for switching default to unbound workqueues from percpu.

- A handshake mechanism was added for canceling BH workers to avoid live
  lock scenarios under PREEMPT_RT.

- Unnecessary rcu_read_lock/unlock() calls were dropped in
  wq_watchdog_timer_fn() and workqueue_congested().

- Documentation was fixed to resolve texinfodocs warnings.

workqueue: fix texinfodocs warning for WQ_* flags reference

Sphinx emitted a warning during make texinfodocs:

  WARNING: Inline literal start-string without end-string.

This was caused by the trailing '*' in "%WQ_*" being parsed as
reStructuredText markup in the kernel-doc comment.

Escape the '*' in the comment so that Sphinx treats it as a literal
character, resolving the warning.

Signed-off-by: Kriish Sharma <kriish.sharma2006@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>