deadlkres() panic

On a recent -current, I got the following panic from deadlkres:

Assertion wchan != NULL failed at /usr/src-nfs/sys/kern/subr_sleepqueue.c:680

Tracing pid 0 tid 100058 td 0xffffff00024bf7a0
kdb_enter() at kdb_enter+0x3d
panic() at panic+0x176
sleepq_type() at sleepq_type+0x56
deadlkres() at deadlkres+0x224
fork_exit() at fork_exit+0x12a
fork_trampoline() at fork_trampoline+0xe
--- trap 0, rip = 0, rsp = 0xffffff8074976d30, rbp = 0 ---
(Hand transcribed, doadump() hung)

deadlkres() came across a TD_IS_SLEEPING()'ing thread that was not a
sleepqueue (ie, td->td_wchan == NULL).

I don't think this is an invalid state for thread to be in: After adding itself
to a sleepq and setting a timeout, the thread calls sleepq_timedwait_sig().
sleepq_catch_signals() determines there is a signal pending so it removes the
thread from the sleepq via sleepq_resume_thread(). Returning to
sleepq_timedwait_sig(), in the call to sleepq_check_timeout(), the thread is
unable to cancel the timeout because it is already firing (likely waiting on
thread_lock()). So the thread calls TD_SET_SLEEPING() followed by mi_switch().
deadlkres() then picks up thread_lock(), finding td is TD_IS_SLEEPING() &&
!TD_ON_SLEEPQ().

The attached patch takes care of the panic for me.