constify d_lookup() arguments

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/dcache.c b/fs/dcache.c
index 93d9f4e1..0e4b5fa 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1889,7 +1889,7 @@
  * dentry is returned. The caller must use dput to free the entry when it has
  * finished using it. %NULL is returned if the dentry does not exist.
  */
-struct dentry *d_lookup(struct dentry *parent, struct qstr *name)
+struct dentry *d_lookup(const struct dentry *parent, const struct qstr *name)
 {
 	struct dentry *dentry;
 	unsigned seq;