fuse: fix "direct_io" private mmap

MAP_PRIVATE mmap could return stale data from the cache for
"direct_io" files.  Fix this by flushing the cache on mmap. 

Found with a slightly modified fsx-linux.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/file.c

@@ -1298,6 +1298,8 @@ static int fuse_direct_mmap(struct file *file, struct vm_area_struct *vma)
 	if (vma->vm_flags & VM_MAYSHARE)
 		return -ENODEV;
 
+	invalidate_inode_pages2(file->f_mapping);
+
 	return generic_file_mmap(file, vma);
 }