[POWERPC] Check for the root node in of_detach_node()

It's not sensible to call of_detach_node() on the root node,
but we should check for it just to be safe.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/prom.c

@@ -1432,6 +1432,8 @@ void of_detach_node(const struct device_node *np)
 	write_lock(&devtree_lock);
 
 	parent = np->parent;
+	if (!parent)
+		goto out_unlock;
 
 	if (allnodes == np)
 		allnodes = np->allnext;
@@ -1455,6 +1457,7 @@ void of_detach_node(const struct device_node *np)
 		prevsib->sibling = np->sibling;
 	}
 
+out_unlock:
 	write_unlock(&devtree_lock);
 }