net: buf: Add more accounting info to net_buf pool

Adding this information to the pool:
* number of available (free) buffers in pool
* total size of the pool in bytes

This can be used when debugging net_buf pool allocations.

Change-Id: I4212fcddb1affdf53e0827c88473d3380e2a4929
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>