vfs: add d_real_inode() helper

Needed by the following fix.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: <stable@vger.kernel.org>

include/linux/dcache.h

@@ -576,5 +576,17 @@ static inline struct inode *vfs_select_inode(struct dentry *dentry,
 	return inode;
 }
 
+/**
+ * d_real_inode - Return the real inode
+ * @dentry: The dentry to query
+ *
+ * If dentry is on an union/overlay, then return the underlying, real inode.
+ * Otherwise return d_inode().
+ */
+static inline struct inode *d_real_inode(struct dentry *dentry)
+{
+	return d_backing_inode(d_real(dentry));
+}
+
 
 #endif	/* __LINUX_DCACHE_H */