net/socket: Use the new net_pkt API to read UDP payload
net/socket: Use the new net_pkt API to read UDP payload

There is no need for appdata pointer: net_pkt's cursor is already at the
right position, i.e. the beginning of the payload right after all IP/UDP
headers.

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

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