enhance receiving path when VF is activated.
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Authored by sepherosa_gmail.com on Aug 11 2017, 5:40 AM.

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Summary

Update hn(4)'s stats properly for non-transparent mode VF.
Allow BPF tapping to hn(4) for non-transparent mode VF.
Don't setup mbuf hash, if 'options RSS' is set.

In Azure, when VF is activated, TCP SYN and SYN|ACK go through hn(4) while the rest of segments and ACKs belonging to the same TCP 4-tuple go through the VF. So don't setup mbuf hash, if a VF is activated and 'options RSS' is not enabled. hn(4) and the VF may use neither the same RSS hash key nor the same RSS hash function, so the hash value for packets belonging to the same flow could be different!
Disable LRO.

hn(4) will only receive broadcast packets, multicast packets, TCP SYN and SYN|ACK (in Azure), LRO is useless for these packet types.

For non-transparent, we definitely _cannot_ enable LRO at all, since the LRO flush will use hn(4) as the receiving interface; i.e. hn_ifp->if_input(hn_ifp, m).

While I'm here, remove unapplied comment and minor style change.