e827091cb1 "block: merge: get the 1st and last bvec via helpers" broken

From: Kent Overstreet
Date: Sat Mar 12 2016 - 02:43:55 EST

I don't know exactly how it's broken, but with that patch segment counting is
broken - I'm seeing blk_rq_map_sg() overrun the end of the sgtable.

I suggest reverting it for 4.5...