resetting needfree
needfree is checked at the very start of arc_reclaim_needed.
This change makes code easier to follow and maintain in face of
potential changed in arc_reclaim_needed.
Also, put the whole sub-block under _KERNEL because needfree can be set
only in kernel code.
To do: rename needfree to something else to aovid confusion with
OpenSolaris global variable of the same name which is used in the same
code, but has different meaning (page deficit).
Note: I have an impression that locking around accesses to this variable
as well as mutual notifications between arc_reclaim_thread and
arc_lowmem are not proper.
MFC after: 1 week
694a0a8717