iommu/io-pgtable-armv7s: Fix kmem_cache_alloc() flags

Whilst the default SLUB allocator happily just merges the original
allocation flags from kmem_cache_create() with those passed through
kmem_cache_alloc(), there is a code path in the SLAB allocator which
will aggressively BUG_ON() if the cache was created with SLAB_CACHE_DMA
but GFP_DMA is not specified for an allocation:

  kernel BUG at mm/slab.c:2536!
  Internal error: Oops - BUG: 0 [#1] SMP ARM
  Modules linked in:[    1.299311] Modules linked in:

  CPU: 1 PID: 1 Comm: swapper/0 Not tainted
  4.5.0-rc6-koelsch-05892-ge7e45ad53ab6795e #2270
  Hardware name: Generic R8A7791 (Flattened Device Tree)
  task: ef422040 ti: ef442000 task.ti: ef442000
  PC is at cache_alloc_refill+0x2a0/0x530
  LR is at _raw_spin_unlock+0x8/0xc
...
  [<c02c6928>] (cache_alloc_refill) from [<c02c6630>] (kmem_cache_alloc+0x7c/0xd4)
  [<c02c6630>] (kmem_cache_alloc) from [<c04444bc>]
  (__arm_v7s_alloc_table+0x5c/0x278)
  [<c04444bc>] (__arm_v7s_alloc_table) from [<c0444e1c>]...