create: do not give chunker a py file object, it is not needed

the os level file handle is enough, the chunker will prefer it if
valid and won't use the file obj, so we can give None there.

this saves these unneeded syscalls:

fstat(5, {st_mode=S_IFREG|0664, st_size=227063, ...}) = 0
ioctl(5, TCGETS, 0x7ffd635635f0)  = -1 ENOTTY (Inappropriate ioctl for device)
lseek(5, 0, SEEK_CUR)             = 0

src/borg/archive.py

@@ -1151,13 +1151,15 @@ class FilesystemObjectProcessors:
                 else:
                     with backup_io('open'):
                         fh = Archive._open_rb(path)
-                    with os.fdopen(fh, 'rb') as fd:
-                        self.process_file_chunks(item, cache, self.stats, self.show_progress, backup_io_iter(self.chunker.chunkify(fd, fh)))
-                        md = self.metadata_collector.stat_attrs(st, path, fd=fh)
-                    if not is_special_file:
-                        # we must not memorize special files, because the contents of e.g. a
-                        # block or char device will change without its mtime/size/inode changing.
-                        cache.memorize_file(path_hash, st, [c.id for c in item.chunks])
+                        try:
+                            self.process_file_chunks(item, cache, self.stats, self.show_progress, backup_io_iter(self.chunker.chunkify(None, fh)))
+                            md = self.metadata_collector.stat_attrs(st, path, fd=fh)
+                        finally:
+                            os.close(fh)
+                        if not is_special_file:
+                            # we must not memorize special files, because the contents of e.g. a
+                            # block or char device will change without its mtime/size/inode changing.
+                            cache.memorize_file(path_hash, st, [c.id for c in item.chunks])
                 self.stats.nfiles += 1
             if md is None:
                 fh = Archive._open_rb(path)