tarfile: addfile() doesn't set member offsets

Bug report

Bug description:

It appears that tarfile.TarFile.addfile() sets neither offset nor offset_data attributes in the TarInfo added to self.members, when it's done adding the file. Members in a reopened TAR have correct values.

Reproducer

from io import BytesIO
from tarfile import TarFile, TarInfo

with TarFile("test.tar", "w") as tar:
    data = b"data"

    tarinfo = TarInfo("test1.txt")
    tarinfo.size = len(data)
    tar.addfile(tarinfo, BytesIO(data))

    tarinfo = TarInfo("test2.txt")
    tarinfo.size = len(data)
    tar.addfile(tarinfo, BytesIO(data))

    for member in tar.getmembers():
        print(member.name, member.offset, member.offset_data)

with TarFile("test.tar", "r") as tar:
    for member in tar.getmembers():
        print(member.name, member.offset, member.offset_data)

Expected output

test1.txt 0 512
test2.txt 1024 1536
test1.txt 0 512
test2.txt 1024 1536

Actual output

test1.txt 0 0
test2.txt 0 0
test1.txt 0 512
test2.txt 1024 1536

CPython versions tested on:

3.15

Operating systems tested on:

Linux

Linked PRs

• gh-150128
• gh-150131
• gh-150278

Start at tarfile.TarFile.addfile() and compare the in-memory TarInfo offsets with the values produced when the archive is reopened. Run the supplied BytesIO reproducer and verify that getmembers() reports the expected offset and offset_data values for both members; review linked PRs gh-150128, gh-150131, and gh-150278 before starting.