snd_dummy: Make callout stopping more robust

If the callout gets rescheduled during detach, we might access freed
pcm_channel resources in dummy_chan_io(), which will cause a panic
similar to this:

panic: ASan: Invalid access, 8-byte read at 0xfffffe00479f65d8, UMAUseAfterFree(fd)
cpuid = 1
time = 1747433047
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0xa5/frame 0xfffffe0046a8d730
kdb_backtrace() at kdb_backtrace+0xc6/frame 0xfffffe0046a8d890
vpanic() at vpanic+0x226/frame 0xfffffe0046a8da30
panic() at panic+0xb5/frame 0xfffffe0046a8db00
kasan_code_name() at kasan_code_name/frame 0xfffffe0046a8dbd0
mtx_lock_flags() at mtx_lock_flags+0xd3/frame 0xfffffe0046a8dcc0
chn_intr() at chn_intr+0x3d/frame 0xfffffe0046a8dce0
dummy_chan_io() at dummy_chan_io+0x9c/frame 0xfffffe0046a8dd10
softclock_call_cc() at softclock_call_cc+0x2bb/frame 0xfffffe0046a8de80
softclock_thread() at softclock_thread+0x162/frame 0xfffffe0046a8def0
fork_exit() at fork_exit+0xa3/frame 0xfffffe0046a8df30
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe0046a8df30

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D50386