dm-crypt: sort writes

Write requests are sorted in a red-black tree structure and are submitted
in the sorted order.

In theory the sorting should be performed by the underlying disk scheduler,
however, in practice the disk scheduler accepts and sorts only 128 requests.
In order to sort more requests, we need to implement our own sorting.

CRs-fixed: 670391
Change-Id: Iffd9345fa1253f5cf2a556893ed36e08f1ac51aa
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Patch-mainline: dm-devel @ 04/05/14, 14:09
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>