block: Don't save/copy bvec array anymore, share when cloning

block: Don't save/copy bvec array anymore, share when cloning

Now that drivers have been converted to the bvec_iter primitives, they
shouldn't be modifying the biovec anymore and thus saving it is
unnecessary - code that was previously making a backup of the bvec array
can now just save bio->bi_iter.

Also, when cloning bios we can usually just reuse the original bio's
bvec array. For code that does need to modify the clone's biovec (the
bounce buffer code, mainly), add bio_clone_biovec().

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Alasdair Kergon <agk@redhat.com>
Cc: dm-devel@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>