rcu: Mark additional concurrent load from ->cpu_no_qs.b.exp

rcu: Mark additional concurrent load from ->cpu_no_qs.b.exp

The per-CPU rcu_data structure's ->cpu_no_qs.b.exp field is updated
only on the instance corresponding to the current CPU, but can be read
more widely.  Unmarked accesses are OK from the corresponding CPU, but
only if interrupts are disabled, given that interrupt handlers can and
do modify this field.

Unfortunately, although the load from rcu_preempt_deferred_qs() is always
carried out from the corresponding CPU, interrupts are not necessarily
disabled.  This commit therefore upgrades this load to READ_ONCE.

Similarly, the diagnostic access from synchronize_rcu_expedited_wait()
might run with interrupts disabled and from some other CPU.  This commit
therefore marks this load with data_race().

Finally, the C-language access in rcu_preempt_ctxt_queue() is OK as
is because interrupts are disabled and this load is always from the
corresponding CPU.  This commit adds a comment giving the rationale for
this access being safe.

This data race was reported by KCSAN.  Not appropriate for backporting
due to failure being unlikely.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>