mm: hugetlbfs: Add some VM_BUG_ON()s to catch non-hugetlbfs pages

Dave Jiang reported that he was seeing oopses when running NUMA systems
and default_hugepagesz=1G.  I traced the issue down to
migrate_page_copy() trying to use the same code for hugetlb pages and
transparent hugepages.  It should not have been trying to pass thp pages
in there.

So, add some VM_BUG_ON()s for the next hapless VM developer that tries
the same thing.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Tested-by: Dave Jiang <dave.jiang@intel.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>