ANDROID: KVM: arm64: Prefault entries when splitting a block mapping

When splitting a block mapping, we install a table entry pointing to an
empty page and recreate the new entries lazily as we fault them in. For
page-tables with the KVM_PGTABLE_S2_IDMAP flag, this can result in
unnecessary translation faults.

When splitting a block for a page-table with KVM_PGTABLE_S2_IDMAP set,
pre-populate the newly allocate page-table page with contiguous ptes
based on the attributes of the block.

Bug: 308373293
Change-Id: I0c53d048de913e193830caef93d75755270db709
Signed-off-by: Will Deacon <willdeacon@google.com>
Signed-off-by: Keir Fraser <keirf@google.com>