net: Make it possible to include net/buf.h even if CONFIG_NET_BUF=n
net: Make it possible to include net/buf.h even if CONFIG_NET_BUF=n

Since CONFIG_NET_BUF_USER_DATA_SIZE was not defined when
CONFIG_NET_BUF=n, compilation would fail on struct net_buf.user_data.

Signed-off-by: Tobias Svehagen <d3302b125bdd383498bbe4afb41749ba75a64b52@gmail.com>

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