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/lib/alloc.c

@@ -10,7 +10,7 @@ void * __init_refok zalloc_maybe_bootmem(size_t size, gfp_t mask)
 {
 	void *p;
 
-	if (mem_init_done)
+	if (slab_is_available())
 		p = kzalloc(size, mask);
 	else {
 		p = memblock_virt_alloc(size, 0);