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

module/zfs/multilist.c

@@ -96,9 +96,12 @@ multilist_create_impl(size_t size, size_t offset,
 }
 
 /*
- * Allocate a new multilist, using the default number of sublists
- * (the number of CPUs, or at least 4, or the tunable
- * zfs_multilist_num_sublists).
+ * Allocate a new multilist, using the default number of sublists (the number
+ * of CPUs, or at least 4, or the tunable zfs_multilist_num_sublists). Note
+ * that the multilists do not expand if more CPUs are hot-added. In that case,
+ * we will have less fanout than boot_ncpus, but we don't want to always
+ * reserve the RAM necessary to create the extra slots for additional CPUs up
+ * front, and dynamically adding them is a complex task.
  */
 multilist_t *
 multilist_create(size_t size, size_t offset,