NOTE about bitmap and volume addresses relations.

All incoming addresses are page-aligned from device start in
snapshot_make_request and filter_write.
Each bitmap block is also page-aligned from device start.
Bitmap addressing doesn't fits volume coordinates for unaligned partitions.
Example for volume_start = 14, volume_nsectors = 19

                  0       8      16
vol               ├───┴───┴───┴───┴──┐
                  │                  │
dev ┌───┬───┬───┬─┴─┬───┬───┬───┬───┬┴──┬───┬
    0       8       16      24      32      40
bm          ┌───────┬───────┬───────┬───────┐
                0       1       2       3

So, for 3-blocks sized volume 4-blocks sized bitmap is required.
volume->bitmap.start_block == volume->start_block == 8
volume->bitmap.last_block == 32
