net/utils: Backup the cursor at relevant place in chksum calculation
net/utils: Backup the cursor at relevant place in chksum calculation

If the family of the packet is not recognized, then do not backup the
cursor.

Signed-off-by: Tomasz Bursztyka <ba81a3a719836727e6857ae83462b9f52ec41006@linux.intel.com>

explora26/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,finikorg/zephyr,explora26/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,ldts/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,ldts/zephyr,explora26/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr