block: Convert bio_iovec() to bvec_iter

For immutable biovecs, we'll be introducing a new bio_iovec() that uses
our new bvec iterator to construct a biovec, taking into account
bvec_iter->bi_bvec_done - this patch updates existing users for the new
usage.

Some of the existing users really do need a pointer into the bvec array
- those uses are all going to be removed, but we'll need the
functionality from immutable to remove them - so for now rename the
existing bio_iovec() -> __bio_iovec(), and it'll be removed in a couple
patches.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: "Ed L. Cashin" <ecashin@coraid.com>
Cc: Alasdair Kergon <agk@redhat.com>
Cc: dm-devel@redhat.com
Cc: "James E.J. Bottomley" <JBottomley@parallels.com>