dm vdo vio-pool: allow variable-sized metadata vios

With larger-sized metadata vio pools, vdo will sometimes need to
issue I/O with a smaller size than the allocated size. Since
vio_reset_bio is where the bvec array and I/O size are initialized,
this reset interface must now specify what I/O size to use.

Signed-off-by: Ken Raeburn <raeburn@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>