Force Device-Mapper to use PAGE_SIZE or smaller I/O when the underlying
device has a bvec_merge_fn routine registered.  This will fix the
situation of Device-Mapper submitting I/Os to RAID-0 that span the
RAID-0 chunk boundaries.

Joe is working on a better solution that actually honors the MD
merge_bvec_fn routine.  But this minimal change will fix the problem for
the time being.