A dead child only loses its slot once a write to it fails, and writes
keep landing in the send buffer until the peer's RST has crossed the
loopback. Fifty back-to-back writes finish in microseconds, so under
full-suite load the loop ran dry before the kernel noticed the close
and the assertion fired. Sleep 5 ms between attempts that have not
reaped yet, bounded at 100 tries: the fast path still exits on the
second write, a loaded machine gets up to half a second.