net: buf: Make net_buf_simple_init() forward compatible
net: buf: Make net_buf_simple_init() forward compatible

Make it safe to call net_buf_simple_init() even if the buffer was
created using the new macros. This is possible to detect since with
the new macros buf->__buf will be non-NULL and with the old
NET_BUF_SIMPLE() macro it will be NULL.

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

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