userns: Convert audit to work with user namespaces enabled

- Explicitly format uids gids in audit messges in the initial user
  namespace. This is safe because auditd is restrected to be in
  the initial user namespace.

- Convert audit_sig_uid into a kuid_t.

- Enable building the audit code and user namespaces at the same time.

The net result is that the audit subsystem now uses kuid_t and kgid_t whenever
possible making it almost impossible to confuse a raw uid_t with a kuid_t
preventing bugs.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric Paris <eparis@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

drivers/tty/tty_audit.c

@@ -69,11 +69,12 @@ static void tty_audit_log(const char *description, struct task_struct *tsk,
 	ab = audit_log_start(NULL, GFP_KERNEL, AUDIT_TTY);
 	if (ab) {
 		char name[sizeof(tsk->comm)];
-		uid_t uid = task_uid(tsk);
+		kuid_t uid = task_uid(tsk);
 
 		audit_log_format(ab, "%s pid=%u uid=%u auid=%u ses=%u "
 				 "major=%d minor=%d comm=", description,
-				 tsk->pid, uid,
+				 tsk->pid,
+				 from_kuid(&init_user_ns, uid),
 				 from_kuid(&init_user_ns, loginuid),
 				 sessionid,
 				 major, minor);