rcu: Make rcu_normal_after_boot writable again

Certain configurations (e.g., systems that make heavy use of netns)
need to use synchronize_rcu_expedited() to service RCU grace periods
even after boot.

Even though synchronize_rcu_expedited() has been traditionally
considered harmful for RT for the heavy use of IPIs, it is perfectly
usable under certain conditions (e.g. nohz_full).

Make rcupdate.rcu_normal_after_boot= again writeable on RT (if NO_HZ_
FULL is defined), but keep its default value to 1 (enabled) to avoid
regressions. Users who need synchronize_rcu_expedited() will boot with
rcupdate.rcu_normal_after_ boot=0 in the kernel cmdline.

Reflect the change in synchronize_rcu_expedited_wait() by removing the
WARN related to CONFIG_PREEMPT_RT.

Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>