Limit calling pmc_hook to when the interrupt comes while running userspace.
We may enable interrupts from within the callback, e.g. in a data abort
during copyin. If we receive an interrupt at that time pmc_hook will be
called again and, as it is handling userspace stack tracing, will hit a
KASSERT as it checks if the trapframe is from userland.

With this I can run hwpmc with intrng on a ThunderX and have it trace all
CPUs.

Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation