selftests/bpf: Drop must_fail from network_helper_opts

selftests/bpf: Drop must_fail from network_helper_opts

The struct member "must_fail" of network_helper_opts() is only used in
cgroup_v1v2 tests, it makes sense to drop it from network_helper_opts.

Return value (fd) of connect_to_fd_opts() and the expect errno (EPERM)
can be checked in cgroup_v1v2.c directly, no need to check them in
connect_fd_to_addr() in network_helpers.c.

This also makes connect_fd_to_addr() function useless. It can be replaced
by connect().

Suggested-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/3faf336019a9a48e2e8951f4cdebf19e3ac6e441.1721282219.git.tanggeliang@kylinos.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>