powerpc: Replace mem_init_done with slab_is_available()

We have a powerpc specific global called mem_init_done which is "set on
boot once kmalloc can be called".

But that's not *quite* true. We set it at the bottom of mem_init(), and
rely on the fact that mm_init() calls kmem_cache_init() immediately
after that, and nothing is running in parallel.

So replace it with the generic and 100% correct slab_is_available().

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/pci-common.c

@@ -76,7 +76,7 @@ struct pci_controller *pcibios_alloc_controller(struct device_node *dev)
 	list_add_tail(&phb->list_node, &hose_list);
 	spin_unlock(&hose_spinlock);
 	phb->dn = dev;
-	phb->is_dynamic = mem_init_done;
+	phb->is_dynamic = slab_is_available();
 #ifdef CONFIG_PPC64
 	if (dev) {
 		int nid = of_node_to_nid(dev);