iommu/arm-smmu: really fix page table locking

Commit a44a9791 ("iommu/arm-smmu: use mutex instead of spinlock for
locking page tables") replaced the page table spinlock with a mutex, to
allow blocking allocations to satisfy lazy mapping requests.

Unfortunately, it turns out that IOMMU mappings are created from atomic
context (e.g. spinlock held during a dma_map), so this change doesn't
really help us in practice.

This patch is a partial revert of the offending commit, bringing back
the original spinlock but replacing our page table allocations for any
levels below the pgd (which is allocated during domain init) with
GFP_ATOMIC instead of GFP_KERNEL.

Cc: <stable@vger.kernel.org>
Reported-by: Andreas Herrmann <andreas.herrmann@calxeda.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>