constify __d_lookup() arguments

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 63e9684..969086c0 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -295,7 +295,7 @@
 /* appendix may either be NULL or be used for transname suffixes */
 extern struct dentry *d_lookup(struct dentry *, struct qstr *);
 extern struct dentry *d_hash_and_lookup(struct dentry *, struct qstr *);
-extern struct dentry *__d_lookup(struct dentry *, struct qstr *);
+extern struct dentry *__d_lookup(const struct dentry *, const struct qstr *);
 extern struct dentry *__d_lookup_rcu(const struct dentry *parent,
 				const struct qstr *name,
 				unsigned *seq, struct inode *inode);