DF-2717 storm: 120s, 3 readers, mode=0xc (1=fchdir-swap 2=relchdir 4=dance 8=readers), A=/tmp/df2717/a B=/home/maxx/df2717/b
DF-2717 storm done in 120.1s
  fchdir swaps       : 0 (0/s)
  relative chdirs    : 0 (0/s)
  volatile dance cyc : 399820 (3329/s)
  reader opens       : 122075241 (1016546/s, enoent 122075241)
RUN_RC=0
guest status after: up   (NO PANIC)

[Dedicated sub-storm isolating the DF-2717 hold-after-drop claim:
399,820 mkdir/chdir/rmdir/chdir-away dance cycles (each ending with
cache_drop of an fd_ncdir-held ncp whose last ref may be dropped ->
kfree_obj) against 1.02M/s reader cache_copy reads for 120 s.
~1.2e8 reader reads of fd_ncdir. Result: clean. No KKASSERT in
_cache_hold, no use-after-free signature.]