selftests: bpf: Less strict size check in sockopt_sk

Originally, the kernel strictly checked the size of the optval in
getsockopt(TCP_ZEROCOPY_RECEIVE) to be equal to sizeof(struct
tcp_zerocopy_receive). With c8856c05, this was changed to allow
optvals of different sizes.

The bpf code in the sockopt_sk test was still performing the strict size
check. This fix adapts the kernel behavior from c8856c05 in the
selftest, i.e., just check if the required fields are there.

Fixes: 9cacf81f

 ("bpf: Remove extra lock_sock for TCP_ZEROCOPY_RECEIVE")
Signed-off-by: Felix Maurer <fmaurer@redhat.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/6f569cca2e45473f9a724d54d03fdfb45f29e35f.1643129402.git.fmaurer@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>