bootmem: use slab if bootmem is no longer available

As a preparation for initializing the slab allocator early, make sure the
bootmem allocator does not crash and burn if someone calls it after slab is up;
otherwise we'd need a flag day for switching to early slab.
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>

mm/bootmem.c

@@ -532,6 +532,9 @@ static void * __init alloc_arch_preferred_bootmem(bootmem_data_t *bdata,
 					unsigned long size, unsigned long align,
 					unsigned long goal, unsigned long limit)
 {
+	if (WARN_ON_ONCE(slab_is_available()))
+		return kzalloc(size, GFP_NOWAIT);
+
 #ifdef CONFIG_HAVE_ARCH_BOOTMEM
 	bootmem_data_t *p_bdata;