rcu: Move expedited grace period (GP) work to RT kthread_worker

Enabling CONFIG_RCU_BOOST did not reduce RCU expedited grace-period
latency because its workqueues run at SCHED_OTHER, and thus can be
delayed by normal processes.  This commit avoids these delays by moving
the expedited GP work items to a real-time-priority kthread_worker.

This option is controlled by CONFIG_RCU_EXP_KTHREAD and disabled by
default on PREEMPT_RT=y kernels which disable expedited grace periods
after boot by unconditionally setting rcupdate.rcu_normal_after_boot=1.

The results were evaluated on arm64 Android devices (6GB ram) running
5.10 kernel, and capturing trace data in critical user-level code.

The table below shows the resulting order-of-magnitude improvements
in synchronize_rcu_expedited() latency:

------------------------------------------------------------------------
|                          |   workqueues  |  kthread_worker |  Diff   |
------------------------------------------------------------------------
| Count                    |          725  |            688  |         |
------------------------------------------------------------------------
| Min Duration       (ns)  |          326  |            447  |  37.12% |
------------------------------------------------------------------------
| Q1                 (ns)  |       39,428  |         38,971  |  -1.16% |
------------------------------------------------------------------------
| Q2 - Median        (ns)  |       98,225  |         69,743  | -29.00% |
------------------------------------------------------------------------
| Q3                 (ns)  |      342,122  |        126,638  | -62.98% |
------------------------------------------------------------------------
| Max Duration       (ns)  |  372,766,967  |      2,329,671  | -99.38% |
------------------------------------------------------------------------
| Avg Duration       (ns)  |    2,746,353  |        151,242  | -94.49% |
------------------------------------------------------------------------
| Standard Deviation (ns)  |   19,327,765  |        294,408  |         |
------------------------------------------------------------------------

The below table show the range of maximums/minimums for
synchronize_rcu_expedited() latency from all experiments:

------------------------------------------------------------------------
|                          |   workqueues  |  kthread_worker |  Diff   |
------------------------------------------------------------------------
| Total No. of Experiments |           25  |             23  |         |
------------------------------------------------------------------------
| Largest  Maximum   (ns)  |  372,766,967  |      2,329,671  | -99.38% |
------------------------------------------------------------------------
| Smallest Maximum   (ns)  |       38,819  |         86,954  | 124.00% |
------------------------------------------------------------------------
| Range of Maximums  (ns)  |  372,728,148  |      2,242,717  |         |
------------------------------------------------------------------------
| Largest  Minimum   (ns)  |       88,623  |         27,588  | -68.87% |
------------------------------------------------------------------------
| Smallest Minimum   (ns)  |          326  |            447  |  37.12% |
------------------------------------------------------------------------
| Range of Minimums  (ns)  |       88,297  |         27,141  |         |
------------------------------------------------------------------------

Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Reported-by: Tim Murray <timmurray@google.com>
Reported-by: Wei Wang <wvw@google.com>
Tested-by: Kyle Lin <kylelin@google.com>
Tested-by: Chunwei Lu <chunweilu@google.com>
Tested-by: Lulu Wang <luluw@google.com>
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>