cxgb4: Add FL DMA mapping error and low counter

Add Free List DMA Mapping Errors to SGE Queue info for
Free Lists. Add Free List "Low" counter to count the number of times we
see the number of pointers that we _think_ the hardware sees in the
Free List below the Egress Threshold.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>