audit: convert PPIDs to the inital PID namespace.

audit: convert PPIDs to the inital PID namespace.

commit c92cdeb45eea38515e82187f48c2e4f435fb4e25 upstream.

sys_getppid() returns the parent pid of the current process in its own pid
namespace.  Since audit filters are based in the init pid namespace, a process
could avoid a filter or trigger an unintended one by being in an alternate pid
namespace or log meaningless information.

Switch to task_ppid_nr() for PPIDs to anchor all audit filters in the
init_pid_ns.

(informed by ebiederman's 6c621b7e)
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
[bwh: Backported to 3.2: sys_getppid() is used by audit_exit() but not
 audit_log_task_info()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>