net: new checksum functions

net: new checksum functions

Introduce new functions to calculate checksums. These new functions
are derivated from the ones provided csumonly.c but slightly reworked.
There is still some room for future optimization of these functions
(maybe SSE/AVX, ...).

This API will be modified in tbe next commits by the introduction of
TSO that requires a different pseudo header checksum to be set in the
packet.

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