audit: Simply AUDIT_TTY_SET and AUDIT_TTY_GET

Use current instead of looking up the current up the current task by
process identifier.  Netlink requests are processed in trhe context of
the sending task so this is safe.

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