eal/freebsd: fix config creation

The config create function did not store the mem config address in
the shared memconfig structure, so the secondary processes couldn't
map it at the required address.

Fixes: b149a7064261 ("eal/freebsd: add config reattach in secondary process")
Cc: stable@dpdk.org
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>