The block layer does not honour bio->bi_size when issuing io, instead
it performs io to the complete bvecs.  This means we have to change
the bio splitting code slightly.

Given a bio we repeatedly apply one of the following three operations
until there is no more io left in the bio:

1) The remaining io does not cross an io/target boundary, so just
   create a clone and issue all of the io.

2) There are some bvecs at the start of the bio that are not split by
   a target boundary.  Create a clone for these bvecs only.

3) The first bvec needs splitting, use bio_alloc() to create *two*
   bios, one for the first half of the bvec, the other for the second
   half.  A bvec can never contain more than one boundary.