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 <johan.hedberg@intel.com>