block: Kill bio_iovec_idx(), __bio_iovec()

bio_iovec_idx() and __bio_iovec() don't have any valid uses anymore -
previous users have been converted to bio_iovec_iter() or other methods.

__BVEC_END() has to go too - the bvec array can't be used directly for
the last biovec because we might only be using the first portion of it,
we have to iterate over the bvec array with bio_for_each_segment() which
checks against the current value of bi_iter.bi_size.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>