posix_acl: Clear SGID bit when setting file permissions

When file permissions are modified via chmod(2) and the user is not in
the owning group or capable of CAP_FSETID, the setgid bit is cleared in
inode_change_ok().  Setting a POSIX ACL via setxattr(2) sets the file
permissions as well as the new ACL, but doesn't clear the setgid bit in
a similar way; this allows to bypass the check in chmod(2).  Fix that.

References: CVE-2016-7097
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/


linux.git
Git-commit: 073931017b49d9458aa351605b43a7e34598caef
Change-Id: Idf7cd8d0fb030fedeabd46254e4c4a9c08bce8b5
[d-cagle@codeaurora.org: Resolve merge conflicts and style]
Signed-off-by: Dennis Cagle <d-cagle@codeaurora.org>