[release-branch.go1.25] archive/tar: limit the number of old GNU sparse format entries

We did not set a limit on the maximum size of sparse maps in
the old GNU sparse format. Set a limit based on the cumulative
size of the extension blocks used to encode the map (consistent
with how we limit the sparse map size for other formats).

Add an additional limit to the total number of sparse file entries,
regardless of encoding, to all sparse formats.

Thanks to Colin Walters (walters@verbum.org),
Uuganbayar Lkhamsuren (https://github.com/uug4na),
and Jakub Ciolek for reporting this issue.

Fixes #78301
Fixes CVE-2026-32288

Change-Id: I84877345d7b41cc60c58771860ba70e16a6a6964
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/3901
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Roland Shoemaker <bracewell@google.com>
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/4003
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/763554
TryBot-Bypass: Gopher Robot <gobot@golang.org>
Auto-Submit: Gopher Robot <gobot@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: David Chase <drchase@google.com>