iommu: Extend LPAE page table format to support custom allocators

We need that in order to implement the VM_BIND ioctl in the GPU driver
targeting new Mali GPUs.

VM_BIND is about executing MMU map/unmap requests asynchronously,
possibly after waiting for external dependencies encoded as dma_fences.
We intend to use the drm_sched framework to automate the dependency
tracking and VM job dequeuing logic, but this comes with its own set
of constraints, one of them being the fact we are not allowed to
allocate memory in the drm_gpu_scheduler_ops::run_job() to avoid this
sort of deadlocks:

- VM_BIND map job needs to allocate a page table to map some memory
  to the VM. No memory available, so kswapd is kicked
- GPU driver shrinker backend ends up waiting on the fence attached to
  the VM map job or any other job fence depending on this VM operation.

With custom allocators, we will be able to pre-reserve enough pages to
guarantee the map/unmap operations we queued will take place without
going through the system allocator. But we can also optimize
allocation/reservation by not free-ing pages immediately, so any
upcoming page table allocation requests can be serviced by some free
page table pool kept at the driver level.

I might also be valuable for other aspects of GPU and similar
use-cases, like fine-grained memory accounting and resource limiting.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Gaurav Kohli <quic_gkohli@quicinc.com>
Tested-by: Gaurav Kohli <quic_gkohli@quicinc.com>
---
v4:
- Add Gaurav's R-b/T-b
v3:
- Don't pass __GFP_ZERO to the custom ->alloc() hook. Returning zeroed
  mem is partof the agreement between the io-pgtable and its user
- Add Robin R-b
v2:
- Add Steven R-b
- Expand on possible use-cases for custom allocators