net/hns3: fix RSS key with null

net/hns3: fix RSS key with null

Since the patch '1848b117' has initialized the variable 'key' in
'struct rte_flow_action_rss' with 'NULL', the PMD will use the
default RSS key when create the first RSS rule with NULL RSS key.
Then, if create a repeated RSS rule with the above, it will not
identify duplicate rules and return an error message.

To solve the preceding problem, determine whether the current RSS keys
are the same based on whether the length of key_len of rss is 0.

Fixes: 1848b117cca1 ("app/testpmd: fix RSS key for flow API RSS rule")
Cc: stable@dpdk.org
Signed-off-by: Lijun Ou <oulijun@huawei.com>