livepatch: use appropriate type for buffer offset variables

As was made noticeable by the last of the commits referenced below,
using a fixed-size type for such purposes is not only against
./CODING_STYLE, but can lead to actual issues. Switch to using size_t
instead, thus also allowing calculations to be lighter-weight in 32-bit
builds.

No functional change for 64-bit builds.

Link: https://gitlab.com/xen-project/xen/-/jobs/7136417308
Fixes: b145b4a ("livepatch: Handle arbitrary size names with the list operation")
Fixes: 5083e0f ("livepatch: Add metadata runtime retrieval mechanism")
Fixes: 43d5c5d ("xen: avoid UB in guest handle arithmetic")
Reported-by: Andrew Cooper <[email protected]>
Signed-off-by: Jan Beulich <[email protected]>
Reviewed-by: Ross Lagerwall <[email protected]>
Release-Acked-by: Oleksii Kurochko <[email protected]>

xen/common/livepatch.c

@@ -1252,7 +1252,7 @@ static int livepatch_list(struct xen_sysctl_livepatch_list *list)
     list->metadata_total_size = 0;
     if ( list->nr )
     {
-        uint64_t name_offset = 0, metadata_offset = 0;
+        size_t name_offset = 0, metadata_offset = 0;
 
         list_for_each_entry( data, &payload_list, list )
         {