arm64: mm: Tidy up force_pte_mapping()

arm64: mm: Tidy up force_pte_mapping()

Tidy up the implementation of force_pte_mapping() to make it easier to
read and introduce the split_leaf_mapping_possible() helper to reduce
code duplication in split_kernel_leaf_mapping() and
arch_kfence_init_pool().

Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Yang Shi <yang@os.amperecomputing.com>
Signed-off-by: Will Deacon <will@kernel.org>