vfs: constify dentry parameter in d_count()

so that it can be used in places like d_compare/d_hash
without causing a compiler warning.
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/dcache.h

@@ -324,7 +324,7 @@ static inline int __d_rcu_to_refcount(struct dentry *dentry, unsigned seq)
 	return ret;
 }
 
-static inline unsigned d_count(struct dentry *dentry)
+static inline unsigned d_count(const struct dentry *dentry)
 {
 	return dentry->d_count;
 }