mm, dax: VMA with vm_ops->pfn_mkwrite wants to be write-notified

For VM_PFNMAP and VM_MIXEDMAP we use vm_ops->pfn_mkwrite instead of
vm_ops->page_mkwrite to notify abort write access.  This means we want
vma->vm_page_prot to be write-protected if the VMA provides this vm_ops.

A theoretical scenario that will cause these missed events is:

  On writable mapping with vm_ops->pfn_mkwrite, but without
  vm_ops->page_mkwrite: read fault followed by write access to the pfn.
  Writable pte will be set up on read fault and write fault will not be
  generated.

I found it examining Dave's complaint on generic/080:

	http://lkml.kernel.org/g/20150831233803.GO3902@dastard

Although I don't think it's the reason.

It shouldn't be a problem for ext2/ext4 as they provide both pfn_mkwrite
and page_mkwrite.

[akpm@linux-foundation.org: add local vm_ops to avoid 80-cols mess]
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Yigal Korman <yigal@plexistor.com>
Acked-by: Boaz Harrosh <boaz@plexistor.com>
Cc: Matthew Wilcox <matthew.r.wilcox@intel.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -1490,13 +1490,14 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
 int vma_wants_writenotify(struct vm_area_struct *vma)
 {
 	vm_flags_t vm_flags = vma->vm_flags;
+	const struct vm_operations_struct *vm_ops = vma->vm_ops;
 
 	/* If it was private or non-writable, the write bit is already clear */
 	if ((vm_flags & (VM_WRITE|VM_SHARED)) != ((VM_WRITE|VM_SHARED)))
 		return 0;
 
 	/* The backer wishes to know when pages are first written to? */
-	if (vma->vm_ops && vma->vm_ops->page_mkwrite)
+	if (vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite))
 		return 1;
 
 	/* The open routine did something to the protections that pgprot_modify