md: dm-req-crypt: Fix memory shrink issue

After FDE enablement data reads/writes uses ICE based encryption
or decryption path, which allocates the memory pools during the
mapping of the requests. This patch cleans up the memory used in
the mapping function after request complete.

Change-Id: I2494ddaca91c92bd61c1482036222796ebceb09c
Signed-off-by: AnilKumar Chimata <anilc@codeaurora.org>