net/mlx5: fix tag ID conflict with sample action

net/mlx5: fix tag ID conflict with sample action

For the flows containing sample action, the tag action was added
implicitly to store the unique flow index into metadata register in the
split prefix subflow, and then match on this index in the split suffix
subflow. The metadata register for flow index of sample split subflows
was also used to store application metadata TAG 0 item, this might cause
TAG 0 corruption in the flows with sample actions.

This patch uses the same metadata register C index as used for
ASO action since it's reserved and not used directly by the application,
and adds the checking in validation to make sure not to conflict
with ASO CT in the same flow.

Fixes: b4c0ddbfcc58 ("net/mlx5: split sample flow into two sub-flows")
Cc: stable@dpdk.org
Signed-off-by: Jiawei Wang <jiaweiw@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>