mm: page_alloc: Make paranoid check in move_freepages a VM_BUG_ON

Since 2.6.24 there has been a paranoid check in move_freepages that looks
up the zone of two pages.  This is a very slow path and the only time I've
seen this bug trigger recently is when memory initialisation was broken
during patch development.  Despite the fact it's a slow path, this patch
converts the check to a VM_BUG_ON anyway as it has served its purpose by
now.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>