mbind: add BUG_ON(!vma) in new_vma_page()

mbind: add BUG_ON(!vma) in new_vma_page()

new_vma_page() is called only by page migration called from do_mbind(),
where pages to be migrated are queued into a pagelist by
queue_pages_range().  queue_pages_range() confirms that a queued page
belongs to some vma, so !vma case is not supposed to be happen.  This
patch adds BUG_ON() to catch this unexpected case.

Signed-off-by: Naoya Horiguchi <[email protected]>
Reported-by: Dan Carpenter <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>