selftests/bpf: Fix bpf-iter-tcp4 test to print correctly the dest IP

Currently, this test is incorrectly printing the destination port in
place of the destination IP.

Fixes: 2767c977 ("selftests/bpf: Implement sample tcp/tcp6 bpf_iter programs")
Signed-off-by: Jose Blanquicet <josebl@microsoft.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210805164044.527903-1-josebl@microsoft.com