Fix race between taskq_destroy and dynamic spawning thread

Fix race between taskq_destroy and dynamic spawning thread

While taskq_destroy would wait for dynamic_taskq to finish its tasks, but it
does not implies the thread being spawned is up and running. This will cause
taskq to be freed before the thread can exit.

We fix this by using tq_nspawn to indicate how many threads are being spawned
before they are inserted to the thread list. And have taskq_destroy to wait
for it to drop to zero.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Issue #553
Closes #550