net: buf: Set default user data size to 4
net: buf: Set default user data size to 4

Since all in-tree users are good with 4 bytes of user data, reduce the
default to 4. Also fix the default and rage values where apparently
'0' and '8' had been swapped.

Signed-off-by: Johan Hedberg <628991c7b0a19c384f9b99c21ec330ce952bb838@intel.com>

explora26/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,ldts/zephyr,finikorg/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,ldts/zephyr,Vudentz/zephyr,ldts/zephyr,mbolivar/zephyr,galak/zephyr,mbolivar/zephyr,galak/zephyr,nashif/zephyr,kraj/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,kraj/zephyr,nashif/zephyr,ldts/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,punitvara/zephyr,punitvara/zephyr,galak/zephyr,zephyriot/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyriot/zephyr,nashif/zephyr,Vudentz/zephyr,punitvara/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,mbolivar/zephyr,nashif/zephyr,mbolivar/zephyr,zephyriot/zephyr,galak/zephyr,explora26/zephyr,kraj/zephyr,Vudentz/zephyr,explora26/zephyr,zephyriot/zephyr,ldts/zephyr