Bluetooth: Mesh: Move per-buffer mesh meta-data to external struct

We've so far been trying to keep the per-buffer mesh meta-data to
a maximum of 8 bytes in anticipation of upcoming net_buf refactoring
that'll have all net_bufs in the system with the same sized user data.

It's however slowly becoming unfeasible to do this - e.g. simply
adding user data to the sending callback would already fill up the
full 8 bytes.

To solve this issue, treat the net_buf user data as just a pointer to
the actual user data, and keep the actual user data in a separate
array. We still keep taking advantage of net_buf_id() however, so
buffers can cheaply be mapped to their meta-data.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>