selftests/bpf: fix the incorrect verification of port numbers.

selftests/bpf: fix the incorrect verification of port numbers.

Check port numbers before calling htons().

According to Dan Carpenter's report, Smatch identified incorrect port
number checks. It is expected that the returned port number is an integer,
with negative numbers indicating errors. However, the value was mistakenly
verified after being translated by htons().

Major changes from v1:

 - Move the variable 'port' to the same line of 'err'.

Fixes: 539c7e67aa4a ("selftests/bpf: Verify that the cgroup_skb filters receive expected packets.")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/bpf/cafd6585-d5a2-4096-b94f-7556f5aa7737@moroto.mountain/
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20230804165831.173627-1-thinker.li@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>