al_eth: Use the common RSS key

[kbowling]

Summary

Fetch the common Toeplitz key instead of filling it with random bytes
during receive-forwarding setup.  Check the key size at compile time and
retain the existing host-to-network conversion for each hardware word.

Leave forwarding, hash selection, and queue mapping unchanged.

MFC after:      2 weeks
Sponsored by:   BBOX.io