The pt hwt(4) backend uses NMIs to receive updates about the latest t
racing buffer offsets from the tracing hardware. However, it uses
taskqueue(9) to schedule the bottom-half handler. This can lead to
a panic since the taskqueue(9) code isn't aware it's being called
from an NMI context and uses the regular scheduling interfaces.
Fix this by scheduling the bottom-half handler using swi(9) and the
SWI_FROMNMI flag.
Fixes: 310162ea218a