net: add function to calculate checksum in mbuf

This function can be used to calculate the checksum of data embedded in
mbuf, that can be composed of several segments.

This function will be used by the virtio pmd in next commits to calculate
the checksum in software in case the protocol is not recognized.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Reviewed-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>