mm: Avoid calling page allocator while in lazy mmu mode

Lazy mmu mode applies to the current task and permits pte modifications
to be deferred and updated at a later time in a batch to improve
performance. tlb_next_batch() is called in lazy mmu mode as follows:

zap_pte_range
  arch_enter_lazy_mmu_mode
  do_zap_pte_range
    zap_present_ptes
      zap_present_folio_ptes
        __tlb_remove_folio_pages
          __tlb_remove_folio_pages_size
            tlb_next_batch
  arch_leave_lazy_mmu_mode

tlb_next_batch() may call into the page allocator which is problematic
with CONFIG_DEBUG_PAGEALLOC because debug_pagealloc_[un]map_pages()
calls the arch implementation of __kernel_map_pages() which must modify
the ptes for the linear map.

There are two possibilities at this point:

- If the arch implementation modifies the ptes directly without first
  entering lazy mmu mode, the pte modifications may get deferred until
  the existing lazy mmu mode is exited. This could result in taking
  spurious faults for example.

- If the arch implementation enters a nested lazy mmu mode before
  modification of the ptes (many arches use apply_to_page_range()),
  then the linear map updates will definitely be applied upon leaving
  the inner lazy mmu mode. But because lazy mmu mode does not support
  nesting, the remainder of the outer user is no longer in lazy mmu
  mode and the optimization opportunity is lost.

So let's just ensure that the page allocator is never called from within
lazy mmu mode. Use the new arch_in_lazy_mmu_mode() API to check if we
are in lazy mmu mode, and if so, when calling into the page allocator,
temporarily leave lazy mmu mode.

Given this new API we can also add VM_WARNings to check that we exit
lazy mmu mode when required to ensure the PTEs are actually updated
prior to tlb flushing.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>