xen/gntdev: do not set VM_PFNMAP

Since we are using the m2p_override we do have struct pages
corresponding to the user vma mmap'ed by gntdev.

Removing the VM_PFNMAP flag makes get_user_pages work on that vma.
An example test case would be using a Xen userspace block backend
(QDISK) on a file on NFS using O_DIRECT.

CC: stable@kernel.org
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/gntdev.c

@@ -722,7 +722,7 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
 	vma->vm_flags |= VM_RESERVED|VM_DONTEXPAND;
 
 	if (use_ptemod)
-		vma->vm_flags |= VM_DONTCOPY|VM_PFNMAP;
+		vma->vm_flags |= VM_DONTCOPY;
 
 	vma->vm_private_data = map;