pkg/directory: Size(): add back type-casts to account for platform differences

I noticed the comment above this code, but didn't see a corresponding type-cast.
Looking at this file's history, I found that these were removed as part of
2f5f0af3fd, which looks to have overlooked some
deliberate type-casts.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/directory/directory_unix.go

@@ -40,12 +40,12 @@ func Size(ctx context.Context, dir string) (size int64, err error) {
 
 		// Check inode to handle hard links correctly
 		inode := fileInfo.Sys().(*syscall.Stat_t).Ino
-		// inode is not a uint64 on all platforms. Cast it to avoid issues.
-		if _, exists := data[inode]; exists {
+		//nolint:unconvert // inode is not an uint64 on all platforms.
+		if _, exists := data[uint64(inode)]; exists {
 			return nil
 		}
-		// inode is not a uint64 on all platforms. Cast it to avoid issues.
-		data[inode] = struct{}{}
+
+		data[uint64(inode)] = struct{}{} //nolint:unconvert // inode is not an uint64 on all platforms.
 
 		size += s