btrfs: raid56: reduce overhead to calculate the bio length

In rbio_update_error_bitmap(), we need to calculate the length of the
rbio.  As since it's called in the endio function, we can not directly
grab the length from bi_iter.

Currently we call bio_for_each_segment_all(), which will always return a
range inside a page.  But that's not necessary as we don't really care
about anything inside the page.

So use bio_for_each_bvec_all(), which can return a bvec across multiple
continuous pages thus reduce the loops.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>