igb: Support for modifying UDP RSS flow hashing

This patch provides ability to enable or disable UDP RSS hashing. It gives
users option of generating RSS hash based on the UDP source and destination
ports numbers. Currently, UDP flow hash is always disabled in igb-driver.

Signed-off-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>