MFC many cxgb and cxgbe features and fixes (r239258, r239259, r239264,
r239266, r239336, r239338, r239339, r239341, r239344, r239514, r239527,
r239528, r239544.

r239258:
Convert some fixed parameters to tunables (with reasonable default
values).

- cong_drop specifies what to do on congestion: nothing, backpressure,
  or drop.
- fl_pktshift specifies the padding before Ethernet payload.
- fl_pad specifies the boundary upto which to pad Ethernet payload.
- spg_len controls the length of the status page.

r239259:
if_iqdrops should include frames truncated within the chip.

r239264:
Assume INET, INET6, and TCP_OFFLOAD when the driver is built out of tree and
KERNBUILDDIR is not set.

r239266:
The size of the buffers in an Ethernet freelist has to be higher than the
interface's MTU.  Initialize such freelists with correct values.

This wasn't a problem for common MTUs (1500 and 9000) as the buffers (2048
and 9216 in size) happened to have enough spare room.  I ran into it when
playing around with unusual MTUs.

r239336:
Allow for a different handler for each type of firmware message.

r239338:
Add a routine (t4_set_tcb_field) to update arbitrary parts of a hardware
TCB.  Filters are programmed by modifying the TCB too (via a different
routine) and the reply to any TCB update is delivered via a
CPL_SET_TCB_RPL.  Figure out whether the reply is for a filter-write or
something else and route it appropriately.

r239339:
Make room for DDP page pods in the default configuration profile.  While
here, bump up the L2 table's size to 4K entries.

r239341:
Initialize various DDP parameters in the main cxgbe(4) driver:

- Setup multiple DDP page sizes.  When the driver attempts DDP it will
  try to combine physically contiguous pages into regions of these sizes.

- Set the indicate size such that the payload carried in the indicate can
  be copied in the header mbuf (and the 16K rx buffer can be recycled).

- Set DDP threshold to the max payload that the chip will coalesce and
  deliver to the driver (this is ~16K by default, which is also why the
  offload rx queue is backed by 16K buffers).  If the chip is able to
  coalesce up to the max it's allowed to, it's a good sign that the peer
  is transmitting in bulk without any TCP PSH.

r239344:
Support for TCP DDP (Direct Data Placement) in the T4 TOE module.

Basically, this is automatic rx zero copy when feasible.  TCP payload is
DMA'd directly into the userspace buffer described by the uio submitted
in soreceive by an application.

- Works with sockets that are being handled by the TCP offload engine
  of a T4 chip (you need t4_tom.ko module loaded after cxgbe, and an
  "ifconfig +toe" on the cxgbe interface).
- Does not require any modification to the application.
- Not enabled by default.  Use hw.t4nex.<X>.toe.ddp="1" to enable it.

r239514:
Minor cleanup: use bitwise ops instead of pointless wrappers around
setbit/clrbit.

r239527:
Cannot hold a mutex around vm_fault_quick_hold_pages, so don't.  Tweak
some comments while here.

r239528:
Avoid a NULL pointer dereference.

r239544:
Deal with the case where a syncache entry added by the TOE driver is
evicted from the syncache but a later syncache_expand succeeds because
of syncookies.  The TOE driver has to resort to more direct means to
install its hooks in the socket in this case.