MFC r250953:
The fasttrap provider cleans up probes asynchronously when a process with
USDT probes exits. This was previously done with a callout; however, it is
possible to sleep while holding the DTrace mutexes, so a panic will occur
on INVARIANTS kernels if the callout handler can't immediately acquire one
of these mutexes. This panic will be frequently triggered on systems where
a USDT-enabled program (perl, for instance) is often run.

This revision changes the fasttrap cleanup mechanism so that a dedicated
thread is used instead of a callout. The old behaviour is otherwise
preserved.

MFC r252493:
Be sure to destory the fasttrap cleanup mutex when unloading the fasttrap
module. This should be MFCed with r250953.