ixv: Report the programmed VF RSS configuration

[kbowling]

Summary

Provide the iflib RSS query methods for X550 and newer VFs, whose RSS key
and hash types are programmed by the VF.  Cache those values and answer
queries under the context lock, without register reads or mailbox
transactions.  Invalidate the snapshot at init, stop, and when an admin
check requests recovery after losing the PF mailbox or detecting reset.
Reject queries while the snapshot or mailbox is unavailable.

Program the common RSS key and cache the values written to the registers.
The key remains stable across resets, including live MTU and capability
changes for which hn can retain its copy without another VF handoff.

Serialize the key in register byte order and report the six supported
IPv4/IPv6 TCP/UDP hash selections from the programmed MRQC, rather than
claiming that every globally requested hash type is enabled.  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

[gallatin]

Why is this needed? All NICs should be using the same rss key these days and that's available via the sysctls in sys/net/rss_config.c

[kbowling]

In D59692#1370212, @gallatin wrote:

Why is this needed? All NICs should be using the same rss key these days and that's available via the sysctls in sys/net/rss_config.c

Principally for hn(4) to match the synthetic and VF settings