mm: check if any page in a pageblock is reserved before marking it MIGRATE_RESERVE

mm: check if any page in a pageblock is reserved before marking it MIGRATE_RESERVE

This fixes a problem where the first pageblock got marked MIGRATE_RESERVE
even though it only had a few free pages.  eg, On current ARM port, The
kernel starts at offset 0x8000 to leave room for boot parameters, and the
memory is freed later.

This in turn caused no contiguous memory to be reserved and frequent
kswapd wakeups that emptied the caches to get more contiguous memory.

Unfortunatelly, ARM needs order-2 allocation for pgd (see
arm/mm/pgd.c#pgd_alloc()).  Therefore the issue is not minor nor easy
avoidable.

[[email protected]: added some explanation]
[[email protected]: add !pfn_valid_within() to check]
[[email protected]: check end_pfn in pageblock_is_reserved]
Signed-off-by: John Stultz <[email protected]>
Signed-off-by: Arve Hjønnevåg <[email protected]>
Signed-off-by: KOSAKI Motohiro <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Acked-by: Dave Hansen <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>