i40e: fix offloading of outer checksum for IPIP tunnel

i40e: fix offloading of outer checksum for IPIP tunnel

When offloading the checksums of ipip tunnels, m->l2_len is set to 0
as there is no tunnel or inner l2 header. Since this is a valid value
remove the test.

By the way, also remove the same test with l3_len because at this
point, it is expected that the software provides proper values in the
mbuf. It should avoid a test in dataplane processing and therefore
slightly increase performance.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Jijiang Liu <jijiang.liu@intel.com>