e1000: Add sysctl for igb(4) DMA Coalesce

This feature can increase efficiency at the expense of latency

It does not work well with the default interrupt delay, but expose
the otherwise unconnected code in the driver in case people want to
experiment.

See
https://www.intel.com/content/dam/support/us/en/documents/network/adapter/pro100/sb/466827_intel_r__dma_coalescing_white_paper_v003.pdf

Sponsored by: Blue Box Systems

(cherry picked from commit c9021340a3266f924bf721182f14f2d265d8a82c)