mempool: fix panic on dump or audit

mempool: fix panic on dump or audit

When doing a mempool dump or an audit, the application can panic because
the length of the cache is greater than the flush threshold, which is
seen as a fatal error. But this can temporarily happen when the mempool
is in use.

Fix the panic condition to abort only when the cache length is greater
than the array.

Fixes: ea5dd2744b90 ("mempool: cache optimisations")
Cc: stable@dpdk.org
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>