i40e: use frame_sz instead of recalculating truesize for building skb

In skb path truesize is calculated while building skb. This is now
avoided and xdp->frame_is used instead for both i40e_build_skb() and
i40e_construct_skb().

Signed-off-by: Tirthendu Sarkar <tirthendu.sarkar@intel.com>
Tested-by: Chandan Kumar Rout <chandanx.rout@intel.com> (A Contingent Worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>