Unconditional kernel panic on flush recursion depth limit instead of documented deferral

Summary

hammer2_flush_core panics with panic(hammer2: flush depth limit) when recursion depth reaches HAMMER2_FLUSH_DEPTH_LIMIT(60) even though documentation at :367 :454-456 :1246 describes deferral-list mechanism. No deferral list exists in hammer2_flush_info. Crafted HAMMER2 image with 60+ nested INDIRECT blockrefs triggers unrecoverable panic on first flush recursing into deep subtree. Mounting requires root privilege but auto-mount scenarios or multi-tenant systems that mount user-supplied images affected. Legitimate trees <10 deep for petabyte-scale so only reachable via crafted input.