selftests/bpf: Make ehash buckets configurable in socket iterator tests

Prepare for bucket resume tests for established TCP sockets by making
the number of ehash buckets configurable. Subsequent patches force all
established sockets into the same bucket by setting ehash_buckets to
one.

Signed-off-by: Jordan Rife <jordan@jrife.io>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>