net: do not do gso for CHECKSUM_UNNECESSARY in netif_needs_gso

This is related to fixing the bug of dropping FCoE frames when disabling tx ip
checksum by 'ethtool -K ethx tx off'. The FCoE protocol stack driver would
use CHECKSUM_UNNECESSARY on tx path instead of CHECKSUM_PARTIAL (as indicated in
the 2/2 of this series). To do so, netif_needs_gso() has to be changed here to
not do gso for both CHECKSUM_PARTIAL and CHECKSUM_UNNECESSARY.

Ref. to original discussion thread:
http://patchwork.ozlabs.org/patch/146567/

Signed-off-by: Yi Zou <yi.zou@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>