net: checksum: Add IP header checksum calculation

At present net_checksum_calculate() only calculates TCP/UDP checksum
in an IP packet, but assumes the IP header checksum to be provided
by the software, e.g.: Linux kernel always calculates the IP header
checksum. However this might not always be the case, e.g.: for an IP
checksum offload enabled stack like VxWorks, the IP header checksum
can be zero.

This adds the checksum calculation of the IP header.

Signed-off-by: Guishan Qin <guishan.qin@windriver.com>
Signed-off-by: Yabing Liu <yabing.liu@windriver.com>
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>