md: fix KW errors

dm-req-crypt is using WARN_ON macro at several places. Using WARN_ON
macro lets code to continue despite detecting an issue. This make
KW unhappy. So return after WARN_ON.

Change-Id: I5a404bb26f89da2a00ecc2dfbd9b2eecc6ba9032
Signed-off-by: Dinesh K Garg <dineshg@codeaurora.org>