dm: Request based dm-crypt

dm-crypt provides bios based device mapper module. dm-crypt
operates on packets with 512 bytes size which is not effiicent
way for HW based crypto blocks. dm-req-crypt is developed to
address this. dm-req-crypt works on requests which carry upto
512KB of data for unmerged requests.

Change-Id: I7d6a63d516dc2dbe80f46c06dd0722847d55bc9f
Signed-off-by: Dinesh K Garg <dineshg@codeaurora.org>