vfs: trivial __d_lookup_rcu() cleanups

These don't change any semantics, but they clean up the code a bit and
mark some arguments appropriately 'const'.

They came up as I was doing the word-at-a-time dcache name accessor
code, and cleaning this up now allows me to send out a smaller relevant
interesting patch for the experimental stuff.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/dcache.c

@@ -104,7 +104,7 @@ static unsigned int d_hash_shift __read_mostly;
 
 static struct hlist_bl_head *dentry_hashtable __read_mostly;
 
-static inline struct hlist_bl_head *d_hash(struct dentry *parent,
+static inline struct hlist_bl_head *d_hash(const struct dentry *parent,
 					unsigned long hash)
 {
 	hash += ((unsigned long) parent ^ GOLDEN_RATIO_PRIME) / L1_CACHE_BYTES;
@@ -1717,8 +1717,9 @@ EXPORT_SYMBOL(d_add_ci);
  * child is looked up. Thus, an interlocking stepping of sequence lock checks
  * is formed, giving integrity down the path walk.
  */
-struct dentry *__d_lookup_rcu(struct dentry *parent, struct qstr *name,
-				unsigned *seq, struct inode **inode)
+struct dentry *__d_lookup_rcu(const struct dentry *parent,
+				const struct qstr *name,
+				unsigned *seqp, struct inode **inode)
 {
 	unsigned int len = name->len;
 	unsigned int hash = name->hash;
@@ -1748,6 +1749,7 @@ struct dentry *__d_lookup_rcu(struct dentry *parent, struct qstr *name,
 	 * See Documentation/filesystems/path-lookup.txt for more details.
 	 */
 	hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {
+		unsigned seq;
 		struct inode *i;
 		const char *tname;
 		int tlen;
@@ -1756,7 +1758,7 @@ struct dentry *__d_lookup_rcu(struct dentry *parent, struct qstr *name,
 			continue;
 
 seqretry:
-		*seq = read_seqcount_begin(&dentry->d_seq);
+		seq = read_seqcount_begin(&dentry->d_seq);
 		if (dentry->d_parent != parent)
 			continue;
 		if (d_unhashed(dentry))
@@ -1771,7 +1773,7 @@ struct dentry *__d_lookup_rcu(struct dentry *parent, struct qstr *name,
 		 * edge of memory when walking. If we could load this
 		 * atomically some other way, we could drop this check.
 		 */
-		if (read_seqcount_retry(&dentry->d_seq, *seq))
+		if (read_seqcount_retry(&dentry->d_seq, seq))
 			goto seqretry;
 		if (unlikely(parent->d_flags & DCACHE_OP_COMPARE)) {
 			if (parent->d_op->d_compare(parent, *inode,
@@ -1788,6 +1790,7 @@ struct dentry *__d_lookup_rcu(struct dentry *parent, struct qstr *name,
 		 * order to do anything useful with the returned dentry
 		 * anyway.
 		 */
+		*seqp = seq;
 		*inode = i;
 		return dentry;
 	}

include/linux/dcache.h

@@ -309,7 +309,8 @@ extern struct dentry *d_ancestor(struct dentry *, struct dentry *);
 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_rcu(struct dentry *parent, struct qstr *name,
+extern struct dentry *__d_lookup_rcu(const struct dentry *parent,
+				const struct qstr *name,
 				unsigned *seq, struct inode **inode);
 
 /**