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 969086c0..03d1692 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -293,7 +293,7 @@
 extern struct dentry *d_ancestor(struct dentry *, struct dentry *);
 
 /* appendix may either be NULL or be used for transname suffixes */
-extern struct dentry *d_lookup(struct dentry *, struct qstr *);
+extern struct dentry *d_lookup(const struct dentry *, const struct qstr *);
 extern struct dentry *d_hash_and_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,