g_up panic, finally with stack trace!

[Andrey Chernov]

This is 10days old kernel panic, the same type I report before, but now I 
use dont-sync-on-panic sysctl, so some stack trace finally available:

Fatal trap 12: page fault while in kernel mode
fault virtual address   = 0x24
fault code              = supervisor read, page not present
instruction pointer     = 0x8:0xc017fdab
stack pointer           = 0x10:0xcd36bb04
frame pointer           = 0x10:0xcd36bb18
code segment            = base 0x0, limit 0xfffff, type 0x1b
			= DPL 0, pres 1, def32 1, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 3 (g_up)
trap number             = 12
panic: page fault

#0  0xc018a4bb in doadump ()
#1  0xc018ab44 in boot ()
#2  0xc018aee8 in panic ()
#3  0xc02992dc in trap_fatal ()
#4  0xc0298983 in trap ()
#5  0xc02897d8 in calltrap ()
#6  0xc01801f9 in _mtx_lock_sleep ()
#7  0xc01d3fac in bufdone ()
#8  0xc01d7e78 in cluster_callback ()
#9  0xc01d3f11 in bufdone ()
#10 0xc01d3d8e in bufdonebio ()
#11 0xc01d3b5c in biodone ()
#12 0xc01520ba in g_dev_done ()
#13 0xc01d3b5c in biodone ()
#14 0xc0154d28 in g_io_schedule_up ()
#15 0xc0154f38 in g_up_procbody ()
#16 0xc0173b51 in fork_exit ()