mempool: introduce helpers for populate and required size

mempool: introduce helpers for populate and required size

Introduce new functions that can used by mempool drivers to
calculate required memory size and to populate mempool.

For now, these helpers just replace the *_default() functions
without change. They will be enhanced in next commit.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Nipun Gupta <nipun.gupta@nxp.com>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>