Implement memory and CPU hotplug

ZFS currently doesn't react to hotplugging cpu or memory into the 
system in any way. This patch changes that by adding logic to the ARC 
that allows the system to take advantage of new memory that is added 
for caching purposes. It also adds logic to the taskq infrastructure 
to support dynamically expanding the number of threads allocated to a 
taskq.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Matthew Ahrens <matthew.ahrens@delphix.com>
Co-authored-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #11212

include/os/linux/spl/sys/taskq.h

@@ -84,6 +84,8 @@ typedef struct taskq {
 	int			tq_nthreads;	/* # of existing threads */
 	int			tq_nspawn;	/* # of threads being spawned */
 	int			tq_maxthreads;	/* # of threads maximum */
+	/* If PERCPU flag is set, percent of NCPUs to have as threads */
+	int			tq_cpu_pct;
 	int			tq_pri;		/* priority */
 	int			tq_minalloc;	/* min taskq_ent_t pool size */
 	int			tq_maxalloc;	/* max taskq_ent_t pool size */
@@ -99,6 +101,9 @@ typedef struct taskq {
 	spl_wait_queue_head_t	tq_work_waitq;	/* new work waitq */
 	spl_wait_queue_head_t	tq_wait_waitq;	/* wait waitq */
 	tq_lock_role_t		tq_lock_class;	/* class when taking tq_lock */
+	/* list node for the cpu hotplug callback */
+	struct hlist_node	tq_hp_cb_node;
+	boolean_t		tq_hp_support;
 } taskq_t;
 
 typedef struct taskq_ent {