There's a chance that thin_generate_damage tries to change ref-counts of
space map blocks due to its random nature, which could lead to problems.
If the ref-counts of metadata space map blocks (shadow source) is changed
to zero, then the ref-counts will become underflow after a shadow operation.
In-place space map modification is a way to prevent that value underflow.
An alternative approach is to avoid changing ref-counts of space map blocks.
