Restore the historical behavior of "sysctl vm.swap_idle_enabled=1". Prior
to r254304, we had separate functions for reclamation and laundering
(vm_pageout_scan) versus updating usage information, i.e., "reference
bits", on active pages (vm_pageout_page_stats), and we only performed
vm_req_vmdaemon(VM_SWAP_IDLE) if vm_pages_needed was true. However, since
r254303, if vm_swap_idle_enabled was "1", we have performed
vm_req_vmdaemon(VM_SWAP_IDLE) regardless of whether we are short of free
pages. This was unintended and too aggressive, so I suspect no one uses
this feature. With this change, we restore the historical behavior and
only perform vm_req_vmdaemon(VM_SWAP_IDLE) when we are short of free
pages.

Reviewed by: kib, markj