mempool: rename element size variables

mempool: rename element size variables

This commit replaces elt_size by total_elt_size when appropriate.

In some mempool functions, we use the size of the elements as arguments
or variables. There is a confusion between the size including or not
including the header and trailer.

To avoid this confusion:
- update the API documentation
- rename the variables and argument names as "elt_size" when the size
  does not include the header and trailer, or else as "total_elt_size".

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked by: Keith Wiles <keith.wiles@intel.com>