net/mlx5: fix L4 protocol validation

net/mlx5: fix L4 protocol validation

- Currently, no device supports partial mask for protocol in IP header.
- As there could be multiple IP items, next_protocol variable in flow
  validation has to be reset for inner layer. Otherwise, inner TCP/UDP
  will see protocol number of outer IP header.
- Remove redundant protocol checking for MPLS, which is done in
  mlx5_flow_validate_item_mpls().

Fixes: 3d69434113d1 ("net/mlx5: add Direct Verbs validation function")
Fixes: 23c1d42c7138 ("net/mlx5: split flow validation to dedicated function")
Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>