From: NeilBrown <neilb@cse.unsw.edu.au>

As no md personalities honour the merge_bvec_fn of underlying devices,
we must make sure never to submit a bio larger than 1 page when a 
merge_bvec_fn is defined.

raid5 already does this (it never submits bios larger than one page).
With this patch, all other raid personalities limit their
max_sectors when a merge_bvec_fn is present.