Allow kicking a taskq to spawn more threads

This patch add a module parameter spl_taskq_kick. When writing non-zero value
to it, it will scan all the taskq, if a taskq contains a task pending for more
than 5 seconds, it will be forced to spawn a new thread. This is use as an
emergency recovery from deadlock, not a general solution.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #529

include/sys/taskq.h

@@ -105,6 +105,7 @@ typedef struct taskq_ent {
 	void			*tqent_arg;
 	taskq_t			*tqent_taskq;
 	uintptr_t		tqent_flags;
+	unsigned long		tqent_birth;
 } taskq_ent_t;
 
 #define	TQENT_FLAG_PREALLOC	0x1