net: icmpv6: Fix echo request packet generation
net: icmpv6: Fix echo request packet generation

`net_ipv6_finalize` expects packet cursor to be reinitialized in order
to finalize packet correctly. This commits adds missing cursor
initialization.

Signed-off-by: Robert Lubos <9d28997c6894a2d0321a7bb399169af7ce99b64e@nordicsemi.no>

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