app/testpmd: fix key for RSS flow rule

app/testpmd: fix key for RSS flow rule

Since the patch '1848b117' has initialized the variable 'key' in
'struct rte_flow_action_rss' with 'NULL', the PMD cannot get the
RSS key now. Details as bellow:

testpmd> flow create 0 ingress pattern eth / ipv4 / end actions
         rss types ipv4-other end key
         1234567890123456789012345678901234567890FFFFFFFFFFFF123
         4567890123456789012345678901234567890FFFFFFFFFFFF
         queues end / end
Flow rule #1 created
testpmd> show port 0 rss-hash key
RSS functions:
         all ipv4-other ip
RSS key:
         4439796BB54C5023B675EA5B124F9F30B8A2C03DDFDC4D02A08C9B3
         34AF64A4C05C6FA343958D8557D99583AE138C92E81150366

This patch sets offset and size of the 'key' variable as the first
parameter of the token 'key'. Later, the address of the RSS key will
be copied to 'key' variable.

Fixes: 1848b117cca1 ("app/testpmd: fix RSS key for flow API RSS rule")
Cc: stable@dpdk.org
Signed-off-by: Alvin Zhang <alvinx.zhang@intel.com>
Tested-by: Jun W Zhou <junx.w.zhou@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>