Use taskq_wait_outstanding() function

Replace taskq_wait() with taskq_wait_oustanding(). This way callers
will only block until previously submitted tasks have been completed.
This was the previous behavior of task_wait() prior to the introduction
of taskq_wait_outstanding() so this isn't really a functionalty change
for these callers.

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