percpu_ref: remove unnecessary RCU grace period for staggered atomic switching confirmation

At the beginning, percpu_ref guaranteed a RCU grace period between a
call to percpu_ref_kill_and_confirm() and the invocation of the
confirmation callback.  This guarantee exposed internal implementation
details and got rescinded while switching over to sched RCU; however,
__percpu_ref_switch_to_atomic() still inserts a full sched RCU grace
period even when it can simply wait for the previous attempt.

Remove the unnecessary grace period and perform the confirmation
synchronously for staggered atomic switching attempts.  Update
comments accordingly.

Signed-off-by: Tejun Heo <tj@kernel.org>