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>
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