test: clean up memory for function reentrancy test

test: clean up memory for function reentrancy test

Function reentrancy test limits maximum number of iterations based
on the number of memzones and cores, however it doesn't free the
memzones after the fact, so on a machine with big amount of cores
the tests will fail due to running out of memzones.

Fix this by introducing cleanup functions for ring and mempool
reentrancy tests.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>