Merge r271647 - 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>
Sponsored by:	Verisign, Inc.
MFC r271648 - 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>
Sponsored by: Verisign, Inc.
MFC r271649 - 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>
Sponsored by: Verisign, Inc.
MFC r271645 - Set DROP_EN on each RX queue if transmit flow-control is disabled.

This allows the NIC to drop frames on the receive queue and not
cause the MAC to block on receiving to _any_ queue.

Tested:

igb0@pci0:5:0:0:        class=0x020000 card=0x152115d9 chip=0x15218086 rev=0x01 hdr=0x00
    vendor     = 'Intel Corporation'
    device     = 'I350 Gigabit Network Connection'
    class      = network
    subclass   = ethernet

Discussed with: Eric Joyner <eric.joyner@intel.com>
MFC after:    1 week
Sponsored by: Norse Corp, Inc.
MFC r271784 - Fix the handling of EOP in status descriptors for if_igb(4)
and don't double-free mbufs.

Like ixgbe(4) chipsets, EOP is only set on the final descriptor
in a chain of descriptors.  So, to free the whole list of descriptors,
we should free the current slot _and_ the assembled list of descriptors
that make up the fragment list.

The existing code was setting discard once it saw EOP + an error status;
it then freed all the subsequent descriptors until the next EOP. That's
totally the wrong order.