s several timer ticks. synchronize_rcu_expedited is
much faster.

With 100Hz timer frequency, when we remove 10000 block devices with
"dmsetup remove_all" command, it takes 27 minutes. With this patch,
removing 10000 block devices takes only 15 seconds.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
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