blk-mq: move blk_mq_update_nr_hw_queues synchronize_rcu call

This fixes the
4.9 backport commit f530afb9
which was
upstream commit f5bbbbe4d63577026f908a809f22f5fd5a90ea1f.

The upstream commit added a call to synchronize_rcu to
_blk_mq_update_nr_hw_queues, just after freezing queues.

In the backport this landed (in blk_mq_update_nr_hw_queues instead),
just after unfreezeing queues.

This commit moves the call to its intended place.

Fixes: f530afb9 ("blk-mq: sync the update nr_hw_queues with blk_mq_queue_tag_busy_iter")
Signed-off-by: Giuliano Procida <gprocida@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>