mm, vmscan: Only allocate and reclaim from zones with pages managed by the buddy allocator

Firmware Assisted Dump (FA_DUMP) on ppc64 reserves substantial amounts
of memory when booting a secondary kernel. Srikar Dronamraju reported that
multiple nodes may have no memory managed by the buddy allocator but still
return true for populated_zone().

Commit 1d82de618ddd ("mm, vmscan: make kswapd reclaim in terms of nodes")
was reported to cause kswapd to spin at 100% CPU usage when fadump was
enabled. The old code happened to deal with the situation of a populated
node with zero free pages by co-incidence but the current code tries to
reclaim populated zones without realising that is impossible.

We cannot just convert populated_zone() as many existing users really
need to check for present_pages. This patch introduces a managed_zone()
helper and uses it in the few cases where it is critical that the check
is made for managed pages -- zonelist constuction and page reclaim.

one nit
s/constuction/construction/

Verified that it works fine.

-- 
Thanks and Regards
Srikar Dronamraju