hash: fix free slot check

In function rte_hash_cuckoo_insert_mw_tm, while looking for
an empty slot, only the first entry in the bucket was being checked,
as key_idx array was not being iterated.

Fixes: 5fc74c2e146d ("hash: check if slot is empty with key index")

Reported-by: Bruce Richardson <bruce.richardson@intel.com>
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>