pt: Switch to swi(9)

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

pt: Switch to swi(9)

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
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D52491

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