Don't pre-create read-only files with --extract --xattrs

[praiskup]

We used to respect the target file mode when pre-creating files in
set_xattr, so we also pre-created read-only files that we were not able
to open later for writing. This is now fixed, and we always create the
file with S_IWUSR.

Fixes the original bug report https://bugzilla.redhat.com/1886540

• src/extract.c (set_xattr): Blindly add S_IWUSR flag to pre-created
  files, to avoid openat failures later.
• tests/xattr08.at: New test-case file.

[praiskup]

https://www.mail-archive.com/bug-tar@gnu.org/msg05958.html

[ljavorsk]

Is there a reason why this patch still hasn't been touched/reviewed?

[praiskup]

Feel free to ping upstream, too. Perhaps this isn't trivial enough to review. Dunno.

[praiskup]

Follow-up discussion:
https://www.mail-archive.com/bug-tar@gnu.org/msg06146.html

Upstream merged 0b74885 and 35d9845.

[praiskup]

This was rather hard to review, but 35d9845 eventually does this:

tar/src/extract.c

Lines 1274 to 1275 in d648fe9

	while (((file_created = set_xattr (file_name, &current_stat_info,
	mode | S_IWUSR, typeflag))

Therefore the new code "pre-creates" the file with S_IWUSR (which was the initial idea about this PR).

[praiskup]

This is documentation only now, proposed here:
https://www.mail-archive.com/bug-tar@gnu.org/msg06216.html

[praiskup]

Applied as e7987b7