block: bio: pass bvec table to bio_init()

block: bio: pass bvec table to bio_init()

Some drivers often use external bvec table, so introduce
this helper for this case. It is always safe to access the
bio->bi_io_vec in this way for this case.

After converting to this usage, it will becomes a bit easier
to evaluate the remaining direct access to bio->bi_io_vec,
so it can help to prepare for the following multipage bvec
support.

Signed-off-by: Ming Lei <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Fixed up the new O_DIRECT cases.

Signed-off-by: Jens Axboe <[email protected]>