mempool: introduce function to get mempool page size

In rte_mempool_populate_default(), we determine the page size,
which is needed for calc_size and allocation of memory.

Move this in a function and export it, it will be used in a next
commit.

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