Initialize and free the DTrace taskqueue in the dtrace module load/unload
handlers rather than in the dtrace device open/close methods. The current
approach can cause a panic if the device is closed which the taskqueue
thread is active, or if a kernel module containing a provider is unloaded
while retained enablings are present and the dtrace device isn't opened.

Submitted by:	gibbs (original version)
Reviewed by:	gibbs
Approved by:	re (glebius)
MFC after:	2 weeks