md: Enable discard option for dm-req-crypt based devices

dm-req-crypt based devices are capable of supporting discard option.
DM layer by default does not enable discard option for DM devices.
Since underlying device supports discard option, mapped target should
also allow it.

Change-Id: Ia540a814c3dcd91f573d1f25d637107a48309a58
Signed-off-by: Dinesh K Garg <dineshg@codeaurora.org>