ice: fix IPIP and SIT TSO offload

The driver was avoiding offload for IPIP (at least) frames due to
parsing the inner header offsets incorrectly when trying to check
lengths.

This length check works for VXLAN frames but fails on IPIP frames
because skb_transport_offset points to the inner header in IPIP
frames, which meant the subtraction of transport_header from
inner_network_header returns a negative value (-20).

With the code before this patch, everything continued to work, but GSO
was being used to segment, causing throughputs of 1.5Gb/s per thread.
After this patch, throughput is more like 10Gb/s per thread for IPIP
traffic.

Fixes: e94d4478

 ("ice: Implement filter sync, NDO operations and bump version")
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Tested-by: Gurucharan G <gurucharanx.g@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>