The structure size did not change:
```
(kgdb) p sizeof(struct callout_cpu)
$1 = 320
```
The DDB command works:
```
# make -C /usr/src/tools/test/callout_free load
callout_free_func = 0xffffffff82c110e0
callout_free_arg = 0xffffffff82c11288
squirrel!
panic: mutex callout_free not owned at /usr/src/sys/kern/kern_mutex.c:202
cpuid = 0
time = 1561739378
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe00004b7870
vpanic() at vpanic+0x19d/frame 0xfffffe00004b78c0
panic() at panic+0x43/frame 0xfffffe00004b7920
unlock_mtx() at unlock_mtx+0x86/frame 0xfffffe00004b7930
softclock_call_cc() at softclock_call_cc+0x175/frame 0xfffffe00004b79e0
softclock() at softclock+0x7c/frame 0xfffffe00004b7a10
ithread_loop() at ithread_loop+0x187/frame 0xfffffe00004b7a70
fork_exit() at fork_exit+0x84/frame 0xfffffe00004b7ab0
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe00004b7ab0
--- trap 0, rip = 0, rsp = 0, rbp = 0 ---
KDB: enter: panic
[ thread pid 12 tid 100029 ]
Stopped at kdb_enter+0x3b: movq $0,kdb_why
db> show callout_last
last callout function: 0xffffffff82c110e0 callout_free_func
last callout argument: 0xffffffff82c11288
last direct callout function: 0xffffffff80c1e730 sleepq_timeout
last direct callout argument: 0xfffff80003e595a0
```