srcu: Yet more detail for srcu_readers_active_idx_check() comments

The comment in srcu_readers_active_idx_check() following the smp_mb()
is out of date, hailing from a simpler time when preemption was disabled
across the bulk of __srcu_read_lock().  The fact that preemption was
disabled meant that the number of tasks that had fetched the old index
but not yet incremented counters was limited by the number of CPUs.

In our more complex modern times, the number of CPUs is no longer a limit.
This commit therefore updates this comment, additionally giving more
memory-ordering detail.

[ paulmck: Apply Nt->Nc feedback from Joel Fernandes. ]

Reported-by: Boqun Feng <[email protected]>
Reported-by: Frederic Weisbecker <[email protected]>
Reported-by: "Joel Fernandes (Google)" <[email protected]>
Reported-by: Neeraj Upadhyay <[email protected]>
Reported-by: Uladzislau Rezki <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

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