tcp lro: use theflowid only when it has hash properties

When a packet is provided to LRO using tcp_lro_queue_mbuf(), a
sequence number is computed based on the m_pkthdr.flowid provided
by he driver. The implicit assumption is that the m_pkthdr.flowid
has hash properties.
The recent use of tcp_lro_queue_mbuf() in iflib exposed a bug in at
least one driver (igc) , which

• reports always that is uses M_HASHTYPE_OPAQUE.
• sets in some cases m_pkthdr.flowid not consistently for packets belonging to the same TCP connection.

This results in severe performance problems for the base TCP stack,
since it handles the packets in the wrong sequence, although they were
received in the correct sequence.
To protect against such misbehaving drivers, just take the
m_pkthdr.flowid only into account, if it has hash properties.
The performance problems were observed by gallatin@ and analyzed
together with rrs@.

Reviewed by: gallatin
Tested by: gallatin
MFC after: 5 Minutes
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D52989