selftests/bpf: Replace tx_prog_fd with tx_prog in test_sockmap

selftests/bpf: Replace tx_prog_fd with tx_prog in test_sockmap

bpf_program__attach_sockmap() needs to take a parameter of type bpf_program
instead of an fd, so tx_prog_fd becomes useless. This patch uses a pointer
tx_prog to point to an item in progs[] array.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/23b37f932c547dd1ebfe154bbc0b0e957be21ee6.1716446893.git.tanggeliang@kylinos.cn