MFC r279243-r279246, r279251, r279691, r279700, and r279701.

MFC r279243-r279246, r279251, r279691, r279700, and r279701.

r279243:
cxgbe(4): request an automatic tx update when a netmap txq idles.

r279244:
cxgbe(4): wait for the hardware to catch up before destroying a netmap txq.

r279245:
cxgbe(4): do not set the netmap rxq interrupts on a hair-trigger.

r279246:
cxgbe(4): set up congestion management for netmap rx queues.

The hw.cxgbe.cong_drop knob controls the response of the chip when
netmap queues are congested.

r279251:
cxgbe(4): allow tx hardware checksumming on the netmap interface.

It is disabled by default but users can set IFCAP_TXCSUM on the
netmap ifnet (ifconfig ncxl0 txcsum) to override netmap and force
the hardware to calculate and insert proper IP and L4 checksums in
outbound frames.

r279691:
cxgbe(4):  provide the correct size of freelists associated with netmap
rx queues to the chip.  This will fix many problems with native netmap
rx on ncxl/ncxgbe interfaces.

r279700:
cxgbe(4): knobs to experiment with the interrupt coalescing timer for
netmap rx queues, and the "batchiness" of rx updates sent to the chip.

These knobs will probably become per-rxq in the near future and will be
documented only after their final form is decided.

r279701:
cxgbe(4): experimental rx packet sink for netmap queues.  This is not
intended for general use.

git-svn-id: svn://svn.freebsd.org/base/stable/10@281253 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f