md: dm-req-crypt: Port missing dm-req-crypt changes to msm-3.18

Some minor changes were missed in dm-req-crypt module as it
escaped from compiling while porting to msm-3.18.
Make minor changes for compatibility with the abstracted bvec
iterators present in recent kernels (see 4f024f37 "block:
Abstract out bvec iterator"). Also remove unnecessary
rq_iterator initializations to avoid compiler warnings resulting
from changes to the definition of that struct in 7988613b
"block: Convert bio_for_each_segment() to bvec_iter".
Also remove usage of now non-existent buffer field of request
structure.

Change-Id: Ife8c46ff8a8348cc29f6296e6f8bb8e90afab19b
Signed-off-by: Dinesh K Garg <dineshg@codeaurora.org>