Sometimes raid0 and linear are required to take a single page bio that
spans two devices.  We use bio_split to split such a bio into two.

The the same time, bio.h is included by linux/raid/md.h so
we don't included it elsewhere anymore.

We also modify the mergeable_bvec functions to allow a bvec
that doesn't fit if it is the first bvec to be added to
the bio, and be careful never to return a negative length from a
bvec_mergable funciton.