md: dm-req-crypt: add message when mapping completed

Add info message when mapping a block-device to
the dm-req-crypt device-mapper target.
This is the first step before this driver can be used for
encryption of a block device.
It is called once per power-up when disk-encryption is enabled.

Change-Id: I51866b95cbe77d1a3d39fcd5d5d5297c78950fa2
Signed-off-by: Amir Samuelov <amirs@codeaurora.org>