Fix a double-free of mbufs in rx_ixgbe_discard().

fmp->buf at the free point is already part of the chain being freed,
so double-freeing is counter-productive.

Submitted by:	Marc De La Gueronniere <mdelagueronniere@verisign.com>
MFC after:	1 week
Sponsored by:	Verisign, Inc.
The error bits are not valid with EOP=0; so intermediary fragments should
not be discarded.

Submitted by:	Marc De La Gueronniere <mdelagueronniere@verisign.com>
MFC after:	1 week
Sponsored by:	Verisign, Inc.
Disable flow-director support until it's been debugged and verified.

The flowdirector feature shares on-chip memory with other things
such as the RX buffers.  In theory it should be configured in a way
that doesn't interfere with the rest of operation.  In practice,
the RX buffer calculation didn't take the flow-director allocation
into account and there'd be overlap.  This lead to various garbage
frames being received containing what looks like internal NIC state.

What _I_ saw was traffic ending up in the wrong RX queues.
If I was doing a UDP traffic test with only one NIC ring receiving
traffic, everything is fine.  If I fired up a second UDP stream
which came in on another ring, there'd be a few percent of traffic
from both rings ending up in the wrong ring.  Ie, the RSS hash would
indicate it was supposed to come in ring X, but it'd come in ring Y.

However, when the allocation was fixed up, the developers at Verisign
still saw traffic stalls.

The flowdirector feature ends up fiddling with the NIC to do various
attempts at load balancing connections by populating flow table rules
based on sampled traffic.  It's likely that all of that has to be
carefully reviewed and made less "magic".

So for now the flow director feature is disabled (which fixes both
what I was seeing and what they were seeing) until it's all much
more debugged and verified.

Tested:

* (me) 82599EB 2x10G NIC, RSS UDP testing.
* (verisign) not sure on the NIC (but likely 82599), 100k-200k/sec TCP
  transaction tests.

Submitted by:	Marc De La Gueronniere <mdelagueronniere@verisign.com>
MFC after:	1 week
Sponsored by:	Verisign, Inc.