rbd: introduce OWN_BVECS data type

If the layout is "fancy", we need to be able to rearrange the provided
bio_vecs in stripe unit chunks to make it possible for the messenger to
read/write directly from/to the provided data buffer, without employing
a temporary data buffer for assembling the result.

Higher level bio_vec arrays are generally immutable, so this requires
copying into a private array.  Only the bio_vecs themselves are shuffled
around, not the actual data.  OWN_BVECS doesn't own any pages.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>