MFC r261533, r261536, r261537, and r263457.

r261533:
cxgbe(4): Use the port's tx channel to identify it to t4_clr_port_stats.

r261536:
cxgbe(4): The T5 allows for a different freelist starvation threshold
for queues with buffer packing.  Use the correct value to calculate a
freelist's low water mark.

r261537:
cxgbe(4): Use the rx channel map (instead of the tx channel map) as the
congestion channel map.

r263457:
cxgbe(4):  Recognize the "spider" configuration where a T5 card's 40G
QSFP port is presented as 4 distinct 10G SFP+ ports to the driver.