mm: make sure all file VMAs have ->vm_ops set

We rely on vma->vm_ops == NULL to detect anonymous VMA: see
vma_is_anonymous(), but some drivers doesn't set ->vm_ops.

As a result we can end up with anonymous page in private file mapping.
That should not lead to serious misbehaviour, but nevertheless is wrong.

Let's fix by setting up dummy ->vm_ops for file mmapping if f_op->mmap()
didn't set its own.

The patch also adds sanity check into __vma_link_rb(). It will help
catch broken VMAs which inserted directly into mm_struct via
insert_vm_struct().

Signed-off-by: Kirill A. Shutemov <[email protected]>
Reviewed-by: Oleg Nesterov <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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