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>

fs/btrfs/raid56.c

@@ -1425,10 +1425,9 @@ static void rbio_update_error_bitmap(struct btrfs_raid_bio *rbio, struct bio *bi
 	int total_sector_nr = get_bio_sector_nr(rbio, bio);
 	u32 bio_size = 0;
 	struct bio_vec *bvec;
-	struct bvec_iter_all iter_all;
 	int i;
 
-	bio_for_each_segment_all(bvec, bio, iter_all)
+	bio_for_each_bvec_all(bvec, bio, i)
 		bio_size += bvec->bv_len;
 
 	/*