rocker: fix neigh tbl index increment race

rocker->neigh_tbl_next_index is used to generate unique indices for neigh
entries programmed into the device.  The way new indices were generated was
racy with the new prepare-commit transaction model.  A simple fix here
removes the race.  The race was with two processes getting the same index,
one process using prepare-commit, the other not:

Proc A					Proc B

PREPARE phase
get neigh_tbl_next_index

					NONE phase
					get neigh_tbl_next_index
					neigh_tbl_next_index++

COMMIT phase
neigh_tbl_next_index++

Both A and B got the same index.  The fix is to store and increment
neigh_tbl_next_index in the PREPARE (or NONE) phase and use value in COMMIT
phase:

Proc A					Proc B

PREPARE phase
get neigh_tbl_next_index
neigh_tbl_next_index++

					NONE phase
					get neigh_tbl_next_index
					neigh_tbl_next_index++

COMMIT phase
// use value stashed in PREPARE phase
Reported-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/rocker/rocker.c

@@ -2901,10 +2901,10 @@ static void _rocker_neigh_add(struct rocker *rocker,
 			      enum switchdev_trans trans,
 			      struct rocker_neigh_tbl_entry *entry)
 {
-	entry->index = rocker->neigh_tbl_next_index;
+	if (trans != SWITCHDEV_TRANS_COMMIT)
+		entry->index = rocker->neigh_tbl_next_index++;
 	if (trans == SWITCHDEV_TRANS_PREPARE)
 		return;
-	rocker->neigh_tbl_next_index++;
 	entry->ref_count++;
 	hash_add(rocker->neigh_tbl, &entry->entry,
 		 be32_to_cpu(entry->ip_addr));