net/sfc: add header segments check for EF100 Tx

net/sfc: add header segments check for EF100 Tx

EF100 native Tx datapath demands that packet header be contiguous
when partial checksum offloads are used since helper function is
used to calculate pseudo-header checksum (and the function requires
contiguous header).

Add an explicit check for this assumption and restructure the code
to avoid TSO header linearisation check since TSO header
linearisation is not done on EF100 native Tx datapath.

Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>