Taskq locking optimizations

Testing has shown that tq->tq_lock can be highly contended when a
large number of small work items are dispatched.  The lock hold time
is reduced by the following changes:

1) Use exclusive threads in the work_waitq

When a single work item is dispatched we only need to wake a single
thread to service it.  The current implementation uses non-exclusive
threads so all threads are woken when the dispatcher calls wake_up().
If a large number of threads are in the queue this overhead can become
non-negligible.

2) Conditionally add/remove threads from work waitq

Taskq threads need only add themselves to the work wait queue if
there are no pending work items.

Signed-off-by: Brian Behlendorf <[email protected]>
Issue #32

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