Flush depth limit panics instead of deferring DoS via deeply nested crafted image

Summary

hammer2_flush_core panics panic(hammer2: flush depth limit) when recursion depth reaches HAMMER2_FLUSH_DEPTH_LIMIT(60) at :584. Header comment :370-373 documents elements should be placed on list for re-execution deferral never implemented. Recovery code implements deferral pattern (HAMMER2_RECOVERY_MAXDEPTH=10 TAILQ list vfsops.c:2281-2292) flush code does not. Crafted image >60 levels single-child indirect nesting causes kernel panic during mount recovery calls hammer2_flush.