selftests: add selftest for UDP SO_PEEK_OFF support

Add the SO_PEEK_OFF selftest for UDP. In this patch, I mainly do
three things:
1. rename tcp_so_peek_off.c
2. adjust for UDP protocol
3. add selftests into it

Suggested-by: Jon Maloy <jmaloy@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Signed-off-by: David S. Miller <davem@davemloft.net>