net/sched: act_mpls: 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 mpls ... index 1
tc filter add ... action mpls index 1

In the current code for act_mpls 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..53
ok 1 a933 - Add MPLS dec_ttl action with pipe opcode
ok 2 08d1 - Add mpls dec_ttl action with pass opcode
ok 3 d786 - Add mpls dec_ttl action with drop opcode
ok 4 f334 - Add mpls dec_ttl action with reclassify opcode
ok 5 29bd - Add mpls dec_ttl action with continue opcode
ok 6 48df - Add mpls dec_ttl action with jump opcode
ok 7 62eb - Add mpls dec_ttl action with trap opcode
ok 8 09d2 - Add mpls dec_ttl action with opcode and cookie
ok 9 c170 - Add mpls dec_ttl action with opcode and cookie of max length
ok 10 9118 - Add mpls dec_ttl action with invalid opcode
ok 11 6ce1 - Add mpls dec_ttl action with label (invalid)
ok 12 352f - Add mpls dec_ttl action with tc (invalid)
ok 13 fa1c - Add mpls dec_ttl action with ttl (invalid)
ok 14 6b79 - Add mpls dec_ttl action with bos (invalid)
ok 15 d4c4 - Add mpls pop action with ip proto
ok 16 91fb - Add mpls pop action with ip proto and cookie
ok 17 92fe - Add mpls pop action with mpls proto
ok 18 7e23 - Add mpls pop action with no protocol (invalid)
ok 19 6182 - Add mpls pop action with label (invalid)
ok 20 6475 - Add mpls pop action with tc (invalid)
ok 21 067b - Add mpls pop action with ttl (invalid)
ok 22 7316 - Add mpls pop action with bos (invalid)
ok 23 38cc - Add mpls push action with label
ok 24 c281 - Add mpls push action with mpls_mc protocol
ok 25 5db4 - Add mpls push action with label, tc and ttl
ok 26 7c34 - Add mpls push action with label, tc ttl and cookie of max length
ok 27 16eb - Add mpls push action with label and bos
ok 28 d69d - Add mpls push action with no label (invalid)
ok 29 e8e4 - Add mpls push action with ipv4 protocol (invalid)
ok 30 ecd0 - Add mpls push action with out of range label (invalid)
ok 31 d303 - Add mpls push action with out of range tc (invalid)
ok 32 fd6e - Add mpls push action with ttl of 0 (invalid)
ok 33 19e9 - Add mpls mod action with mpls label
ok 34 1fde - Add mpls mod action with max mpls label
ok 35 0c50 - Add mpls mod action with mpls label exceeding max (invalid)
ok 36 10b6 - Add mpls mod action with mpls label of MPLS_LABEL_IMPLNULL (invalid)
ok 37 57c9 - Add mpls mod action with mpls min tc
ok 38 6872 - Add mpls mod action with mpls max tc
ok 39 a70a - Add mpls mod action with mpls tc exceeding max (invalid)
ok 40 6ed5 - Add mpls mod action with mpls ttl
ok 41 77c1 - Add mpls mod action with mpls ttl and cookie
ok 42 b80f - Add mpls mod action with mpls max ttl
ok 43 8864 - Add mpls mod action with mpls min ttl
ok 44 6c06 - Add mpls mod action with mpls ttl of 0 (invalid)
ok 45 b5d8 - Add mpls mod action with mpls ttl exceeding max (invalid)
ok 46 451f - Add mpls mod action with mpls max bos
ok 47 a1ed - Add mpls mod action with mpls min bos
ok 48 3dcf - Add mpls mod action with mpls bos exceeding max (invalid)
ok 49 db7c - Add mpls mod action with protocol (invalid)
ok 50 b070 - Replace existing mpls push action with new ID
ok 51 95a9 - Replace existing mpls push action with new label, tc, ttl and cookie
ok 52 6cce - Delete mpls pop action
ok 53 d138 - Flush mpls actions

Fixes: 2a2ea508

 ("net: sched: add mpls manipulation actions to TC")
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>