dm_lookup_exception
struct dm_exception *e = dm_lookup_exception(&s->pending, chunk);
e = dm_lookup_exception(&s->complete, chunk);
e = dm_lookup_exception(&s->complete, chunk);
e = dm_lookup_exception(&s->complete, chunk);
e = dm_lookup_exception(&snap->complete, chunk);
e = dm_lookup_exception(&snap->complete, chunk);
e = dm_lookup_exception(&s->complete, old_chunk);