Add bio_get_nr_vecs(). It returns an approximate number of pages that
can be added to a block device. It's just a ballpark number, but I think
this is quite fine for the type of thing it is needed for: mpage etc
need to know an approx size of a bio that they need to allocate. It
would be silly to continously allocate 64-page sized bio_vec entries, if
the target cannot do more than 8, for example.