selftest: bpf: Remove mssind boundary check in test_tcp_custom_syncookie.c.

Smatch reported a possible off-by-one in tcp_validate_cookie().

However, it's false positive because the possible range of mssind is
limited from 0 to 3 by the preceding calculation.

  mssind = (cookie & (3 << 6)) >> 6;

Now, the verifier does not complain without the boundary check.
Let's remove the checks.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/bpf/6ae12487-d3f1-488b-9514-af0dac96608f@stanley.mountain/


Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240821013425.49316-1-kuniyu@amazon.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>