Provide SIOCGIFRSSKEY and SIOCGIFRSSHASH for X550 and newer VFs, whose RSS
key and hash types are programmed by the VF.  Cache those values and
answer queries under the iflib context lock, without register reads or
mailbox transactions.  Invalidate the snapshot at init and stop, and
reject queries while mailbox readiness is lost.

Choose the VF-owned key once at attachment and reuse it when programming
RSS after a reset.  hn can retain its copy across live MTU or capability
changes, so generating a new key at each init would leave the synthetic
and VF receive paths using different keys until the next handoff.

Serialize the key in register byte order and report the six supported
IPv4/IPv6 TCP/UDP hash selections from the programmed MRQC.  A single
receive queue still performs Toeplitz hashing.  Older VFs retain
EOPNOTSUPP because their PF-owned RSS settings are not available here.
This allows hn to synchronize synthetic RSS with the actual VF settings
instead of suppressing otherwise usable receive hash metadata.

MFC after:      2 weeks
Sponsored by:   BBOX.io