mempool: clarify default populate function

mempool: clarify default populate function

No functional change.
Clarify the populate function to make future changes easier to
understand.

Rename the variables:
- to avoid negation in the name
- to have more understandable names

Remove useless variable (no_pageshift is equivalent to pg_sz == 0).

Remove duplicate affectation of "external" variable.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>