Update a comment to reflect new taskq internals

As of the removal of the taskq work list made in commit:

commit 2c02b71b1411176905228666abf7a50a2e5f85dc
Author: Prakash Surya <surya1@llnl.gov>
Date:   Mon Dec 5 17:32:48 2011 -0800

    Replace tq_work_list and tq_threads in taskq_t

    To lay the ground work for introducing the taskq_dispatch_prealloc()
    interface, the tq_work_list and tq_threads fields had to be replaced
    with new alternatives in the taskq_t structure.

the comment above taskq_wait_check has been incorrect. This change is an
attempt at bringing that description more in line with the current
implementation. Essentially, references to the old task work list had to
be updated to reference the new taskq thread active list.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #65