net: utils: Rework the IP packet checksum calculation

Instead of directly setting pointer to where to start to calculate
the various IP related checksums, use the net_frag_skip() to first
find out what is the fragment where the calculation should start.

This needs to be like this so that if the IP header + possible
extension are so long that they do not fit the first fragment,
we need to be prepared to look into second fragment.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>