i just managed to get the dump.
(kgdb) bt
#0 __curthread() at /usr/src/sys/amd64/include/pcpu_aux.h:55
#1 dump_savectx() at /usr/src/sys/kern/kern_shutdown.c:394
#2 0xffffffff80c01138in dumpsys(di=0x0) at
/usr/src/sys/x86/include/dump.h:87
#3 doadump(textdump=<optimized out>) at
/usr/src/sys/kern/kern_shutdown.c:423
#4 kern_reboot(howto=260) at /usr/src/sys/kern/kern_shutdown.c:497
#5 0xffffffff80c015aein vpanic(fmt=<optimized out>,
ap=ap@entry=0xfffffe003615fa50) at /usr/src/sys/kern/kern_shutdown.c:930
#6 0xffffffff80c013e3in panic(fmt=<unavailable>) at
/usr/src/sys/kern/kern_shutdown.c:854
#7 0xffffffff81090d47in trap_fatal(frame=0xfffffe003615fb40, eva=1088)
at /usr/src/sys/amd64/amd64/trap.c:940
#8 0xffffffff81090d9fin trap_pfault(frame=0xfffffe003615fb40,
usermode=false, signo=<optimized out>, ucode=<optimized out>) at
/usr/src/sys/amd64/amd64/trap.c:759
#9 <signal handler called>
#10 __mtx_lock_sleep(c=0xfffff805abba8900, v=<optimized out>) at
/usr/src/sys/kern/kern_mutex.c:594
#11 0xffffffff8315d6d4in bstp_ifupdstatus(arg=0xfffff80011effa18,
pending=pending@entry=0) at /usr/src/sys/net/bridgestp.c:1823
#12 0xffffffff8315d3c6in bstp_linkstate(bp=0xfffff805abba8900,
bp@entry=0xfffff80011effa18) at /usr/src/sys/net/bridgestp.c:1802
#13 0xffffffff83158f25in bridge_linkstate(ifp=0xfffff8000432e800) at
/usr/src/sys/net/if_bridge.c:3729
#14 0xffffffff80d1310ein do_link_state_change(arg=0xfffff8000432e800,
pending=1) at /usr/src/sys/net/if.c:2330
#15 0xffffffff80c63401in
taskqueue_run_locked(queue=queue@entry=0xfffff80003ae3800) at
/usr/src/sys/kern/subr_taskqueue.c:514
#16 0xffffffff80c631b8in taskqueue_run(queue=0xfffff80003ae3800) at
/usr/src/sys/kern/subr_taskqueue.c:529
#17 0xffffffff80bc143ain
intr_event_execute_handlers(ie=0xfffff80003ae3700, p=<optimized out>) at
/usr/src/sys/kern/kern_intr.c:1167
#18 ithread_execute_handlers(ie=0xfffff80003ae3700, p=<optimized out>)
at /usr/src/sys/kern/kern_intr.c:1180
#19 ithread_loop(arg=arg@entry=0xfffff80003af50e0) at
/usr/src/sys/kern/kern_intr.c:1268
#20 0xffffffff80bbe1dein fork_exit(callout=0xffffffff80bc11e0
<ithread_loop>, arg=0xfffff80003af50e0, frame=0xfffffe003615ff40) at
/usr/src/sys/kern/kern_fork.c:1105
#21 <signal handler called>
#22 mi_startup() at /usr/src/sys/kern/init_main.c:322
