selftests/bpf: Free strdup memory in test_sockmap

The strdup() function returns a pointer to a new string which is a
duplicate of the string "ptr". Memory for the new string is obtained
with malloc(), and need to be freed with free().

This patch adds these missing "free(ptr)" in check_whitelist() and
check_blacklist() to avoid memory leaks in test_sockmap.c.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/b76f2f4c550aebe4ab8ea73d23c4cbe4f06ea996.1714374022.git.tanggeliang@kylinos.cn