Fix __d_drop() to properly initialize the d_hash fields,

so that __d_drop() can safely be done multiple times on
a dentry without corrupting other hash entries.

Noticed by Trond Myklebust.

include/linux/dcache.h

@@ -175,7 +175,7 @@ extern spinlock_t dcache_lock;
 static inline void __d_drop(struct dentry *dentry)
 {
 	dentry->d_vfs_flags |= DCACHE_UNHASHED;
-	hlist_del_rcu(&dentry->d_hash);
+	hlist_del_rcu_init(&dentry->d_hash);
 }
 
 static inline void d_drop(struct dentry *dentry)