dm crypt: use unbound workqueue for request processing

Use unbound workqueue by default so that work is automatically balanced
between available CPUs.  The original behavior of encrypting using the
same cpu that IO was submitted on can still be enabled by setting the
optional 'same_cpu_crypt' table argument.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
(cherry picked from commit f3396c58fd8442850e759843457d78b6ec3a9589)
Signed-off-by: Alex Shi <alex.shi@linaro.org>