md: dm-req-crypt does not release the device

dm-req-crypt does not release the device which it got during
construction of dm-req-crypt based node. This causes issue if
dm-req-crypt based device is created and destroyed without
rebooting the device.

Change-Id: Ifeb1210a6e1cf365b8a656556082806a24f3e582
Signed-off-by: Dinesh K Garg <dineshg@codeaurora.org>