Add geneve Rx offload support for x550em_a.

The implementation follows the vxlan code with the lower 16 bits of
the VXLANCTRL register holding the UDP port for VXLAN and the upper
for Geneve.

Disabled NFS filters in the RFCTL register which allows us to simplify
the check for VXLAN and Geneve packets in ixgbe_rx_checksum().

Removed vxlan from the name of the callback functions and replaced it
with udp_tunnel which is more in line with the new API.
Signed-off-by: NEmil Tantilov <emil.s.tantilov@intel.com>
Tested-by: NAndrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>