dm-crypt: use unbound workqueue for request processing

Use unbound workqueue so that work is automatically ballanced between
available CPUs.

CRs-fixed: 670391
Change-Id: I169099d0b5b27535633c9d3aaab2037b5fea6aa9
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
[joonwoop@codeaurora.org: resolve trivial merge conflict]
Patch-mainline: dm-devel @ 04/05/14, 14:06
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>