net: buf: convert `#define` to `static inline`
net: buf: convert `#define` to `static inline`

Swap the `net_buf_*` -> `net_buf_simple_*` conversion `#define`s to
`static inline` functions. This is to provide better type checking on
the use of these functions.

Signed-off-by: Jordan Yates <d74a90e280bbad1cc3646781a4e234b467b60fe1@data61.csiro.au>

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