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/mem.c

@@ -61,7 +61,6 @@
 #define CPU_FTR_NOEXECUTE	0
 #endif
 
-int mem_init_done;
 unsigned long long memory_limit;
 
 #ifdef CONFIG_HIGHMEM
@@ -377,8 +376,6 @@ void __init mem_init(void)
 	pr_info("  * 0x%08lx..0x%08lx  : vmalloc & ioremap\n",
 		VMALLOC_START, VMALLOC_END);
 #endif /* CONFIG_PPC32 */
-
-	mem_init_done = 1;
 }
 
 void free_initmem(void)