From Marcus Alanen <maalanen@ra.abo.fi>

Don't retake the zone lock after spilling a batch of pages into the
buddy.

Instead, just clear local variable `zone' to indicate that no lock is
held.

This is actually a common case - whenever release_pages() is called
with exactly 16 pages (truncate, page reclaim..) Marcus' patch will
save a lock and an unlock.

Also, remove some lock-avoidance heuristics in
pagevec_deactivate_inactive(): the caller has already made these
checks, and the chance of the check here actually doing anything useful
is negligible.