mm: compaction: acquire the zone->lock as late as possible

Compaction's free scanner acquires the zone->lock when checking for
PageBuddy pages and isolating them.  It does this even if there are no
PageBuddy pages in the range.

This patch defers acquiring the zone lock for as long as possible.  In the
event there are no free pages in the pageblock then the lock will not be
acquired at all which reduces contention on zone->lock.

Change-Id: Iaf8b30f94e43b03ac93d751b7b97dbe0b76b87af
Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Richard Davies <richard@arachsys.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: Avi Kivity <avi@redhat.com>
Acked-by: Rafael Aquini <aquini@redhat.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Tested-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Git-commit: f40d1e42bb988d2a26e8e111ea4c4c7bac819b7e
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>