fuse: check create mode in atomic open

Verify that the VFS is passing us a complete create mode with the S_IFREG to
atomic open.

Reported-by: Steve <steveamigauk@yahoo.co.uk>
Reported-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Tested-by: Richard W.M. Jones <rjones@redhat.com>

fs/fuse/dir.c

@@ -383,6 +383,9 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
 	struct fuse_entry_out outentry;
 	struct fuse_file *ff;
 
+	/* Userspace expects S_IFREG in create mode */
+	BUG_ON((mode & S_IFMT) != S_IFREG);
+
 	forget = fuse_alloc_forget();
 	err = -ENOMEM;
 	if (!forget)