bpf: no need to nullify ri->map in xdp_do_redirect

We are guaranteed to have a NULL ri->map in this branch since
we test for it earlier, so we don't need to reset it here.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>