net/sched: act_sample: fix action bind logic

The TC architecture allows filters and actions to be created independently.
In filters the user can reference action objects using:
tc action add action sample ... index 1
tc filter add ... action pedit index 1

In the current code for act_sample this is broken as it checks netlink
attributes for create/update before actually checking if we are binding to an
existing action.

tdc results:
1..29
ok 1 9784 - Add valid sample action with mandatory arguments
ok 2 5c91 - Add valid sample action with mandatory arguments and continue control action
ok 3 334b - Add valid sample action with mandatory arguments and drop control action
ok 4 da69 - Add valid sample action with mandatory arguments and reclassify control action
ok 5 13ce - Add valid sample action with mandatory arguments and pipe control action
ok 6 1886 - Add valid sample action with mandatory arguments and jump control action
ok 7 7571 - Add sample action with invalid rate
ok 8 b6d4 - Add sample action with mandatory arguments and invalid control action
ok 9 a874 - Add invalid sample action without mandatory arguments
ok 10 ac01 - Add invalid sample action without mandatory argument rate
ok 11 4203 - Add invalid sample action without mandatory argument group
ok 12 14a7 - Add invalid sample action without mandatory argument group
ok 13 8f2e - Add valid sample action with trunc argument
ok 14 45f8 - Add sample action with maximum rate argument
ok 15 ad0c - Add sample action with maximum trunc argument
ok 16 83a9 - Add sample action with maximum group argument
ok 17 ed27 - Add sample action with invalid rate argument
ok 18 2eae - Add sample action with invalid group argument
ok 19 6ff3 - Add sample action with invalid trunc size
ok 20 2b2a - Add sample action with invalid index
ok 21 dee2 - Add sample action with maximum allowed index
ok 22 560e - Add sample action with cookie
ok 23 704a - Replace existing sample action with new rate argument
ok 24 60eb - Replace existing sample action with new group argument
ok 25 2cce - Replace existing sample action with new trunc argument
ok 26 59d1 - Replace existing sample action with new control argument
ok 27 0a6e - Replace sample action with invalid goto chain control
ok 28 3872 - Delete sample action with valid index
ok 29 a394 - Delete sample action with invalid index

Fixes: 5c5670fa ("net/sched: Introduce sample tc action")
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>