arc_prune_task uses a refcount to protect arc_prune_t, but it doesn't prevent
the underlying zsb from disappearing if there's a concurrent umount. We fix
this by force the caller of arc_remove_prune_callback to wait for
arc_prune_taskq to finish.
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#4687Closes#4690
4442f60d8e