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/mm/pgtable_64.c

@@ -231,7 +231,7 @@ void __iomem * __ioremap_caller(phys_addr_t addr, unsigned long size,
 	if ((size == 0) || (paligned == 0))
 		return NULL;
 
-	if (mem_init_done) {
+	if (slab_is_available()) {
 		struct vm_struct *area;
 
 		area = __get_vm_area_caller(size, VM_IOREMAP,
@@ -315,7 +315,7 @@ void __iounmap(volatile void __iomem *token)
 {
 	void *addr;
 
-	if (!mem_init_done)
+	if (!slab_is_available())
 		return;
 	
 	addr = (void *) ((unsigned long __force)