FROMLIST: rcu: Don't deboost before reporting expedited quiescent state

Currently rcu_preempt_deferred_qs_irqrestore() releases rnp->boost_mtx
before reporting the expedited quiescent state.  Under heavy real-time
load, this can result in this function being preempted before the
quiescent state is reported, which can in turn prevent the expedited grace
period from completing.  Tim Murray reports that the resulting expedited
grace periods can take hundreds of milliseconds and even more than one
second, when they should normally complete in less than a millisecond.

This patch follows Neeraj's suggestion (seconded by Tim and by Uladzislau
Rezki) of simply reversing the two operations.

Reported-by: Tim Murray <timmurray@google.com>
Reported-by: Joel Fernandes <joelaf@google.com>
Reported-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Bug: 217236054
Link: https://lore.kernel.org/lkml/20220204232406.814-9-paulmck@kernel.org/


Signed-off-by: Tim Murray <timmurray@google.com>
Change-Id: Ic0f0330b65a0c776a563e03e4b59bf4ab4fbccf6