slab: Handle ARCH_DMA_MINALIGN correctly

James Hogan hit boot problems in next-20130204 on Meta:

  META213-Thread0 DSP [LogF] kobject (4fc03980): tried to init an initialized object, something is seriously wrong.
  META213-Thread0 DSP [LogF]
  META213-Thread0 DSP [LogF] Call trace:
  META213-Thread0 DSP [LogF] [<4000888c>] _show_stack+0x68/0x7c
  META213-Thread0 DSP [LogF] [<400088b4>] _dump_stack+0x14/0x28
  META213-Thread0 DSP [LogF] [<40103794>] _kobject_init+0x58/0x9c
  META213-Thread0 DSP [LogF] [<40103810>] _kobject_create+0x38/0x64
  META213-Thread0 DSP [LogF] [<40103eac>] _kobject_create_and_add+0x14/0x8c
  META213-Thread0 DSP [LogF] [<40190ac4>] _mnt_init+0xd8/0x220
  META213-Thread0 DSP [LogF] [<40190508>] _vfs_caches_init+0xb0/0x160
  META213-Thread0 DSP [LogF] [<401851f4>] _start_kernel+0x274/0x340
  META213-Thread0 DSP [LogF] [<40188424>] _metag_start_kernel+0x58/0x6c
  META213-Thread0 DSP [LogF] [<40000044>] __start+0x44/0x48
  META213-Thread0 DSP [LogF]
  META213-Thread0 DSP [LogF] devtmpfs: initialized
  META213-Thread0 DSP [LogF] L2 Cache: Not present
  META213-Thread0 DSP [LogF] BUG: failure at fs/sysfs/dir.c:736/sysfs_read_ns_type()!
  META213-Thread0 DSP [LogF] Kernel panic - not syncing: BUG!
  META213-Thread0 DSP [Thread Exit] Thread has exited - return code = 4294967295

And bisected the problem to commit 95a05b42

 ("slab: Common constants for
kmalloc boundaries").

As it turns out, a fixed KMALLOC_SHIFT_LOW does not work for arches with
higher alignment requirements.

Determine KMALLOC_SHIFT_LOW from ARCH_DMA_MINALIGN instead.

Reported-and-tested-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>