dm-crypt: fix warning in shutdown path

The counter for the number of allocated pages includes pages in the
mempool's reserve, so checking that the number of allocated pages is 0
needs to happen after we exit the mempool.

Fixes: 6f1c819c ("dm: convert to bioset_init()/mempool_init()")
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Reported-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Mike Snitzer <snitzer@redhat.com>

Fixed to always just use percpu_counter_sum()

Signed-off-by: Jens Axboe <axboe@kernel.dk>