selftests/bpf: Add rto max for bpf_setsockopt test

Test the TCP_RTO_MAX_MS optname in the existing setget_sockopt test.

Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250219081333.56378-3-kerneljasonxing@gmail.com