Add taskq_wait_outstanding() function

SPL commit behlendorf/spl@9cef1b5 adds the taskq_wait_outstanding()
interface. See the commit log for the full justification for this
addition. This patch adds the required user space counterpart.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>