selftests/bpf: Fix rare segfault in sock_fields prog test

test_sock_fields__detach() got called with a null pointer here when one
of the CHECKs or ASSERTs up to the test_sock_fields__open_and_load()
call resulted in a jump to the "done" label.

A skeletons *__detach() is not safe to call with a null pointer, though.
This led to a segfault.

Go the easy route and only call test_sock_fields__destroy() which is
null-pointer safe and includes detaching.

Came across this while looking[1] to introduce the usage of
bpf_tcp_helpers.h (included in progs/test_sock_fields.c) together with
vmlinux.h.

[1] https://lore.kernel.org/bpf/629bc069dd807d7ac646f836e9dca28bbc1108e2.camel@mailbox.tu-berlin.de/

Fixes: 8f50f16f ("selftests/bpf: Extend verifier and bpf_sock tests for dst_port loads")
Signed-off-by: Jörn-Thorben Hinz <jthinz@mailbox.tu-berlin.de>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Reviewed-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20220621070116.307221-1-jthinz@mailbox.tu-berlin.de

tools/testing/selftests/bpf/prog_tests/sock_fields.c

@@ -394,7 +394,6 @@ void serial_test_sock_fields(void)
 	test();
 
 done:
-	test_sock_fields__detach(skel);
 	test_sock_fields__destroy(skel);
 	if (child_cg_fd >= 0)
 		close(child_cg_fd);