Linux: Cleanup taskq threads spawn/exit

This changes taskq_thread_should_stop() to limit maximum exit rate
for idle threads to one per 5 seconds.  I believe the previous one
was broken, not allowing any thread exits for tasks arriving more
than one at a time and so completing while others are running.

Also while there:
 - Remove taskq_thread_spawn() calls on task allocation errors.
 - Remove extra taskq_thread_should_stop() call.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #15873

man/man4/spl.4

@@ -186,18 +186,8 @@ reading it could cause a lock-up if the list grow too large
 without limiting the output.
 "(truncated)" will be shown if the list is larger than the limit.
 .
-.It Sy spl_taskq_thread_timeout_ms Ns = Ns Sy 10000 Pq uint
-(Linux-only)
-How long a taskq has to have had no work before we tear it down.
-Previously, we would tear down a dynamic taskq worker as soon
-as we noticed it had no work, but it was observed that this led
-to a lot of churn in tearing down things we then immediately
-spawned anew.
-In practice, it seems any nonzero value will remove the vast
-majority of this churn, while the nontrivially larger value
-was chosen to help filter out the little remaining churn on
-a mostly idle system.
-Setting this value to
-.Sy 0
-will revert to the previous behavior.
+.It Sy spl_taskq_thread_timeout_ms Ns = Ns Sy 5000 Pq uint
+Minimum idle threads exit interval for dynamic taskqs.
+Smaller values allow idle threads exit more often and potentially be
+respawned again on demand, causing more churn.
 .El