net/sfc: support TSO in EF10 Tx datapath

net/sfc: support TSO in EF10 Tx datapath

Implementation includes following limitations:

1) Packet's header length must be less than 256 (SFC_TSOH_STD_LEN);
2) Offset of the TCP header must be less than 208
   (EF10_TCP_HEADER_OFFSET_LIMIT);
3) Number of Tx descriptors must be not less than number of descriptors
   needed for TSO settings plus header plus one data segment.

If above conditions are not met, the packet is dropped.

If the maximum descriptor space is insufficient to hold entire TSO packet,
only a part of the packet is sent.

Signed-off-by: Igor Romanov <igor.romanov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>