[$] Preserving the mobility of ZONE_MOVABLE

Memory fragmentation has long been a problem for Linux systems, to thepoint that, for years, finding even two physically contiguous pages was anuncertain affair. That said, the situation has improved considerably inthe last decade or so thanks to a number of changes implemented by thememory-management developers. One of those changes is the creation of"movable"memory zones where pages can be relocated if need be. All that work is fornothing, though, if somebody comes along and pins down a page in one ofthese movable zones. Thispatch set from Pavel Tatashin seeks to prevent that from happening, butmay risk creating problems elsewhere.