ixgbe: support TCP segmentation offload

ixgbe: support TCP segmentation offload

Implement TSO (TCP segmentation offload) in ixgbe driver. The driver is
now able to use PKT_TX_TCP_SEG mbuf flag and mbuf hardware offload infos
(l2_len, l3_len, l4_len, tso_segsz) to configure the hardware support of
TCP segmentation.

In ixgbe, when doing TSO, the IP length must not be included in the TCP
pseudo header checksum. A new function ixgbe_fix_tcp_phdr_cksum() is
used to fix the pseudo header checksum of the packet before giving it to
the hardware.

In the patch, the tx_desc_cksum_flags_to_olinfo() and
tx_desc_ol_flags_to_cmdtype() functions have been reworked to make them
clearer. This should not impact performance as gcc (version 4.8 in my
case) is smart enough to convert the tests into a code that does not
contain any branch instruction.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>