take write_seqcount_invalidate() into __d_drop()

... and reorder it with making d_unhashed() true.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -470,30 +470,29 @@ static void dentry_lru_add(struct dentry *dentry)
  */
 static void ___d_drop(struct dentry *dentry)
 {
-	if (!d_unhashed(dentry)) {
-		struct hlist_bl_head *b;
-		/*
-		 * Hashed dentries are normally on the dentry hashtable,
-		 * with the exception of those newly allocated by
-		 * d_obtain_root, which are always IS_ROOT:
-		 */
-		if (unlikely(IS_ROOT(dentry)))
-			b = &dentry->d_sb->s_roots;
-		else
-			b = d_hash(dentry->d_name.hash);
+	struct hlist_bl_head *b;
+	/*
+	 * Hashed dentries are normally on the dentry hashtable,
+	 * with the exception of those newly allocated by
+	 * d_obtain_root, which are always IS_ROOT:
+	 */
+	if (unlikely(IS_ROOT(dentry)))
+		b = &dentry->d_sb->s_roots;
+	else
+		b = d_hash(dentry->d_name.hash);
 
-		hlist_bl_lock(b);
-		__hlist_bl_del(&dentry->d_hash);
-		hlist_bl_unlock(b);
-		/* After this call, in-progress rcu-walk path lookup will fail. */
-		write_seqcount_invalidate(&dentry->d_seq);
-	}
+	hlist_bl_lock(b);
+	__hlist_bl_del(&dentry->d_hash);
+	hlist_bl_unlock(b);
 }
 
 void __d_drop(struct dentry *dentry)
 {
-	___d_drop(dentry);
-	dentry->d_hash.pprev = NULL;
+	if (!d_unhashed(dentry)) {
+		___d_drop(dentry);
+		dentry->d_hash.pprev = NULL;
+		write_seqcount_invalidate(&dentry->d_seq);
+	}
 }
 EXPORT_SYMBOL(__d_drop);
 
@@ -2853,9 +2852,10 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
 	write_seqcount_begin_nested(&target->d_seq, DENTRY_D_LOCK_NESTED);
 
 	/* unhash both */
-	/* ___d_drop does write_seqcount_barrier, but they're OK to nest. */
-	___d_drop(dentry);
-	___d_drop(target);
+	if (!d_unhashed(dentry))
+		___d_drop(dentry);
+	if (!d_unhashed(target))
+		___d_drop(target);
 
 	/* Switch the names.. */
 	if (exchange)