igb: Make TSO check for CHECKSUM_PARTIAL to avoid skb_is_gso check

This change is meant to reduce the overhead for workloads that are not
using either TSO or checksum offloads.  Most of the time the compiler
should jump ahead after failing this check to the VLAN check since in the
igb_tx_csum call we start with that check as well.

Signed-off-by: Alexander Duyck <alexander.h.duyck@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>