net: buf: Add net_buf_id() API

Add a net_buf_id() API which translates a buffer into a zero-based
index, based on its placement in the buffer pool. This can be useful
if you want to associate an external array of meta-data contexts with
the buffers of a pool.

The added value of this API is slightly limited at the moment, since
the net_buf API allows custom user-data sizes for each pool (i.e. the
user data can be used instead of a separately allocated meta-data
array). However, there's some refactoring coming soon which will unify
all net_buf structs to have the same fixed (and typically small)
amount of user data. In such cases it may be desirable to have
external user data in order not to inflate all buffers in the system
because of a single pool needing the extra memory.

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