btrfs: simplify bvec iteration in index_one_bio()

Flatten the two loops by open coding bio_for_each_segment() and advancing
the iterator one sector at a time.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Qu Wenruo <wqu@suse.com>
[ Fix a bug that @offset is not increased. ]
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>