mm: reduce the amount of work done when updating min_free_kbytes

mm: reduce the amount of work done when updating min_free_kbytes

Orabug: 14073214
When min_free_kbytes is updated, some pageblocks are marked
MIGRATE_RESERVE.  Ordinarily, this work is unnoticable as it happens early
in boot but on large machines with 1TB of memory, this has been reported
to delay boot times, probably due to the NUMA distances involved.

The bulk of the work is due to calling calling pageblock_is_reserved() an
unnecessary amount of times and accessing far more struct page metadata
than is necessary.  This patch significantly reduces the amount of work
done by setup_zone_migrate_reserve() improving boot times on 1TB machines.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 938929f14cb595f43cd1a4e63e22d36cab1e4a1f)

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>