ovl: fix may_write_real() for overlayfs directories

Overlayfs directory file_inode() is the overlay inode whether the real
inode is upper or lower.

This fixes a regression in xfstest generic/158.

Fixes: 7c6893e3c9ab ("ovl: don't allow writing ioctl on lower layer")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/namespace.c

@@ -468,7 +468,9 @@ static inline int may_write_real(struct file *file)
 
 	/* File refers to upper, writable layer? */
 	upperdentry = d_real(dentry, NULL, 0, D_REAL_UPPER);
-	if (upperdentry && file_inode(file) == d_inode(upperdentry))
+	if (upperdentry &&
+	    (file_inode(file) == d_inode(upperdentry) ||
+	     file_inode(file) == d_inode(dentry)))
 		return 0;
 
 	/* Lower layer: can't write to real file, sorry... */