Revert "slab: remove BAD_ALIEN_MAGIC"

This reverts commit a6406168 ("slab: remove BAD_ALIEN_MAGIC").

commit a6406168 ("slab: remove BAD_ALIEN_MAGIC") assumes that the
system with !CONFIG_NUMA has only one memory node.  But, it turns out to
be false by the report from Geert.  His system, m68k, has many memory
nodes and is configured in !CONFIG_NUMA.  So it couldn't boot with above
change.

Here goes his failure report.

  With latest mainline, I'm getting a crash during bootup on m68k/ARAnyM:

  enable_cpucache failed for radix_tree_node, error 12.
  kernel BUG at /scratch/geert/linux/linux-m68k/mm/slab.c:1522!
  *** TRAP #7 ***   FORMAT=0
  Current process id is 0
  BAD KERNEL TRAP: 00000000
  Modules linked in:
  PC: [<0039c92c>] kmem_cache_init_late+0x70/0x8c
  SR: 2200  SP: 00345f90  a2: 0034c2e8
  d0: 0000003d    d1: 00000000    d2: 00000000    d3: 003ac942
  d4: 00000000    d5: 00000000    a0: 0034f686    a1: 0034f682
  Process swapper (pid: 0, task=0034c2e8)
  Frame format=0
  Stack from 00...