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/rtas.c

@@ -401,7 +401,7 @@ static char *__fetch_rtas_last_error(char *altbuf)
 			buf = altbuf;
 		} else {
 			buf = rtas_err_buf;
-			if (mem_init_done)
+			if (slab_is_available())
 				buf = kmalloc(RTAS_ERROR_LOG_MAX, GFP_ATOMIC);
 		}
 		if (buf)
@@ -461,7 +461,7 @@ int rtas_call(int token, int nargs, int nret, int *outputs, ...)
 
 	if (buff_copy) {
 		log_error(buff_copy, ERR_TYPE_RTAS_LOG, 0);
-		if (mem_init_done)
+		if (slab_is_available())
 			kfree(buff_copy);
 	}
 	return ret;