Add TASKQ_DYNAMIC feature

Setting the TASKQ_DYNAMIC flag will create a taskq with dynamic
semantics.  Initially only a single worker thread will be created
to service tasks dispatched to the queue.  As additional threads
are needed they will be dynamically spawned up to the max number
specified by 'nthreads'.  When the threads are no longer needed,
because the taskq is empty, they will automatically terminate.

Due to the low cost of creating and destroying threads under Linux
by default new threads and spawned and terminated aggressively.
There are two modules options which can be tuned to adjust this
behavior if needed.

* spl_taskq_thread_sequential - The number of sequential tasks,
without interruption, which needed to be handled by a worker
thread before a new worker thread is spawned.  Default 4.

* spl_taskq_thread_dynamic - Provides the ability to completely
disable the use of dynamic taskqs on the system.  This is provided
for the purposes of debugging and troubleshooting.  Default 1
(enabled).

This behavior is fundamentally consistent with the dynamic taskq
implementation found in both illumos and FreeBSD.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #458

man/man5/spl-module-parameters.5

@@ -249,3 +249,37 @@ where a thread should run.
 .sp
 Default value: \fB0\fR
 .RE
+
+.sp
+.ne 2
+.na
+\fBspl_taskq_thread_dynamic\fR (int)
+.ad
+.RS 12n
+Allow dynamic taskqs.  When enabled taskqs which set the TASKQ_DYNAMIC flag
+will by default create only a single thread.  New threads will be created on
+demand up to a maximum allowed number to facilitate the completion of
+outstanding tasks.  Threads which are no longer needed will be promptly
+destroyed.  By default this behavior is enabled but it can be disabled to
+aid performance analysis or troubleshooting.
+.sp
+Default value: \fB1\fR
+.RE
+
+.sp
+.ne 2
+.na
+\fBspl_taskq_thread_sequential\fR (int)
+.ad
+.RS 12n
+The number of items a taskq worker thread must handle without interruption
+before requesting a new worker thread be spawned.  This is used to control
+how quickly taskqs ramp up the number of threads processing the queue.
+Because Linux thread creation and destruction are relatively inexpensive a
+small default value has been selected.  This means that normally threads will
+be created aggressively which is desirable.  Increasing this value will
+result in a slower thread creation rate which may be preferable for some
+configurations.
+.sp
+Default value: \fB4\fR
+.RE