mm/slab: fix unalignment problem on Malta with EVA due to slab merge

Unlike SLUB, sometimes, object isn't started at the beginning of the
slab in SLAB.  This causes the unalignment problem after slab merging is
supported by commit 12220dea ("mm/slab: support slab merge").

Following is the report from Markos that fail to boot on Malta with EVA.

    Calibrating delay loop... 19.86 BogoMIPS (lpj=99328)
    pid_max: default: 32768 minimum: 301
    Mount-cache hash table entries: 4096 (order: 0, 16384 bytes)
    Mountpoint-cache hash table entries: 4096 (order: 0, 16384 bytes)
    Kernel bug detected[#1]:
    CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.17.0-05639-g12220dea07f1 #1631
    task: 1f04f5d8 ti: 1f050000 task.ti: 1f050000
    epc   : 80141190 alloc_unbound_pwq+0x234/0x304
        Not tainted
    ra    : 80141184 alloc_unbound_pwq+0x228/0x304
    Process swapper/0 (pid: 1, threadinfo=1f050000, task=1f04f5d8, tls=00000000)
    Call Trace:
      alloc_unbound_pwq+0x234/0x304
      apply_workqueue_attrs+0x11c/0x294
      __alloc_workqueue_key+0x23c/0x470
      init_workqueues+0x320/0x400
      do_one_initcall+0xe8/0x23c
      kernel_init_freeable+0x9c/0x224
      kernel_init+0x10/0x100
      ret_from_kernel_thread+0x14/0x1c
    [ end trace cb88537fdc8fa200 ]
    Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b

alloc_unbound_pwq() allocates slab object from pool_workqueue.  This
kmem_cache requires 256 bytes alignment, but, current merging code
doesn't honor that, and merge it with kmalloc-256.  kmalloc-256 requires
only cacheline size alignment so that above failure occurs.  However, in
x86, kmalloc-256 is luckily aligned in 256 bytes, so the problem didn't
happen on it.

To fix this problem, this patch introduces alignment mismatch check in
find_mergeable().  This will fix the problem.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Reported-by: Markos Chandras <Markos.Chandras@imgtec.com>
Tested-by: Markos Chandras <Markos.Chandras@imgtec.com>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>