remove the poor emulation of the IllumOS needfree global variable to prevent
the ARC reclaim thread running longer than needed.

Update the arc::needfree dtrace probe triggered in arc_lowmem() to also report
the value we may want to free.

Submitted by: Nikita Kozlov <nikita.kozlov at blade-group.com>
Reviewed by: avg
Approved by: avg
MFC after: 3 weeks
Sponsored by: blade
Differential Revision: https://reviews.freebsd.org/D12163