arm64/mm: Implement pte_batch_hint()

When core code iterates over a range of ptes and calls ptep_get() for
each of them, if the range happens to cover contpte mappings, the number
of pte reads becomes amplified by a factor of the number of PTEs in a
contpte block. This is because for each call to ptep_get(), the
implementation must read all of the ptes in the contpte block to which
it belongs to gather the access and dirty bits.

This causes a hotspot for fork(), as well as operations that unmap
memory such as munmap(), exit and madvise(MADV_DONTNEED). Fortunately we
can fix this by implementing pte_batch_hint() which allows their
iterators to skip getting the contpte tail ptes when gathering the batch
of ptes to operate on. This results in the number of PTE reads returning
to 1 per pte.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Tested-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>