selftests/bpf: Adjust data size to have ETH_HLEN

The function bpf_test_init() now returns an error if user_size
(.data_size_in) is less than ETH_HLEN, causing the tests to
fail. Adjust the data size to ensure it meets the requirement of
ETH_HLEN.

Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250121150643.671650-2-syoshida@redhat.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>