DF-2584 fix-side control — PATCHED kernel (#1 Oct 14), crafted depth 10
(deferral never engages: 10 < limit 16; exercises the plain write path)
BEFORE 526817f23b663472746e94a93ad3beae637fc2d69a0d2b857176eb93e2cf8750  (original random)
dd 32KB zeros + sync:
SYNC-OK CACHED c35020473aed1b4642cd726cad727b63fff2824ad68cedd7ffb73c7cbd890479  (32KB zeros)
umount + remount:
REMOUNTED c35020473aed1b4642cd726cad727b63fff2824ad68cedd7ffb73c7cbd890479  (PERSISTED)
=> write path + flush persist correctly for non-deferred deep chains.