Fix bug with -x --xattr read-only files

Problem reported by Kevin Raymond in:
https://bugzilla.redhat.com/show_bug.cgi?id=1886540
* src/extract.c (open_output_file): If we already created the
empty file, do not open with O_EXCL, or with O_CREAT or O_TRUNC
for that matter.  Instead, use only O_NOFOLLOW to avoid some
races.  When estimating current mode, use openflag & O_EXCL rather
than overwriting_old_files.
(extract_file): Also invert S_IWUSR if it’s not set.
* tests/xattr08.at: New test.
* tests/Makefile.am, tests/testsuite.at: Add it.

tests/Makefile.am

@@ -275,6 +275,7 @@ TESTSUITE_AT = \
  xattr05.at\
  xattr06.at\
  xattr07.at\
+ xattr08.at\
  acls01.at\
  acls02.at\
  acls03.at\

tests/testsuite.at

@@ -502,6 +502,7 @@ m4_include([xattr04.at])
 m4_include([xattr05.at])
 m4_include([xattr06.at])
 m4_include([xattr07.at])
+m4_include([xattr08.at])
 
 m4_include([acls01.at])
 m4_include([acls02.at])

tests/xattr08.at

@@ -0,0 +1,41 @@
+# Process this file with autom4te to create testsuite. -*- Autotest -*-
+#
+# Test suite for GNU tar.
+# Copyright 2022 Free Software Foundation, Inc.
+
+# This file is part of GNU tar.
+
+# GNU tar is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+
+# GNU tar is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+# Test description:
+# Test read-only files can be extracted with --xattr.
+# Per report:
+# https://lists.gnu.org/r/bug-tar/2020-10/msg00001.html
+
+AT_SETUP([xattrs: xattrs and read-only files])
+AT_KEYWORDS([xattrs xattr08])
+
+AT_TAR_CHECK([
+AT_XATTRS_PREREQ
+mkdir dir dir2
+genfile --file dir/file
+
+setfattr -n user.test -v OurDirValue dir/file
+chmod a-w dir/file
+
+tar --xattrs -C dir -cf archive.tar file
+tar --xattrs -C dir2 -xf archive.tar
+])
+
+AT_CLEANUP