KVM: arm64: Constify start/end/phys fields of the pgtable walker data

As we are revamping the way the pgtable walker evaluates some of the
data, make it clear that we rely on somew of the fields to be constant
across the lifetime of a walk.

For this, flag the start, end and phys fields of the walk data as
'const', which will generate an error if we were to accidentally
update these fields again.
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/hyp/pgtable.c

@@ -58,9 +58,9 @@
 struct kvm_pgtable_walk_data {
 	struct kvm_pgtable_walker	*walker;
 
-	u64				start;
+	const u64			start;
 	u64				addr;
-	u64				end;
+	const u64			end;
 };
 
 static bool kvm_phys_is_valid(u64 phys)
@@ -352,7 +352,7 @@ int kvm_pgtable_get_leaf(struct kvm_pgtable *pgt, u64 addr,
 }
 
 struct hyp_map_data {
-	u64				phys;
+	const u64			phys;
 	kvm_pte_t			attr;
 };
 
@@ -578,7 +578,7 @@ void kvm_pgtable_hyp_destroy(struct kvm_pgtable *pgt)
 }
 
 struct stage2_map_data {
-	u64				phys;
+	const u64			phys;
 	kvm_pte_t			attr;
 	u8				owner_id;