This changes the way we do pool lookups when freeing a bio.  Right now
we use bi_max as a handle into bvec_array[], to find the pool where it
came from.  This used to be just fine, because if you had a private bio,
you could specify your own destructor.  But now we have bio_add_page()
which also needs to know where the bio came from, or more precisely, it
needs to know how many entries the bio can hold.

So I've changed bi_max to bi_max_vecs, it now contains the number of vec
entries in the bio. Privately allocated bio's (or on stack) can now just
set bio->bi_max_vecs to reflect the max size. The pool index for the
default destructor is stored in the top bits of bi_flags.