md: Enable cache flush for dm-req-crypt based devices

dm-req-crypt based devices may be capable of cache flushing. DM
layer by default does not enable cache flushing for DM devices.
If underlying device supports flush, mapped target should also
allow it.

Change-Id: Ifbaa02411155cc49076cf3c8087851dc00a877a0
Signed-off-by: Dinesh K Garg <dineshg@codeaurora.org>