hash: retrieve a key given its position

hash: retrieve a key given its position

The function rte_hash_get_key_with_position is added in this patch.
As the position returned when adding a key is frequently used as an
offset into an array of user data, this function performs the operation
of retrieving a key given this offset.

A possible use case would be to delete a key from the hash table when
its entry in the array of data has certain value. For instance, the key
could be a flow 5-tuple, and the value stored in the array a time
stamp.

Signed-off-by: Juan Antonio Montesinos <juan.antonio.montesinos.delgado@ericsson.com>
Signed-off-by: Yari Adan Petralanda <yari.adan.petralanda@ericsson.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>