panic at in_pcbremlists()

A couple of days ago I posted a message about panicing whenever
shutting down inetd.  The panic persists, but now it seems to happen
at any time, always stopping at in_pcbremlists().

The kernel is up-to-date, a kernel built on July 30th runs stable as a
rock.

Fatal trap 12: page fault while in kernel mode
fault virtual address   = 0x10001
fault code              = supervisor read, page not present
instruction pointer     = 0x8:0xc02e044a
stack pointer           = 0x10:0xd68d9c08
frame pointer           = 0x10:0xd68d9c14
code  segment           = base 0x0 limit 0xffff, type 0x1b
                        = DPL 0, pres 1, def32 1,gran 1
processor eflags        = interrupt enabled, resume, IOPL=0
current process         = 11 (swi7: tty:sio clock)
trap number             = 12
panic page fault

DDB says:
Stopped at in_pcbremlists+0x8d

stack trace:

in_pcbremlists()
in_pcbdetach()
tcp_twclose()
tcp_timer
tcp_slowtimeo()
pfslowtimeo()
softclock()
ithread_loop()
fork_exit()
fork_trampoline()