net/mlx5e: Use a memory barrier to enforce PTP WQ xmit submission tracking occurs after populating the metadata_map

In the Linux kernel, the following vulnerability has been resolved:

net/mlx5e: Use a memory barrier to enforce PTP WQ xmit submission tracking occurs after populating the metadata_map

Just simply reordering the functions mlx5e_ptp_metadata_map_put and

mlx5e_ptpsq_track_metadata in the mlx5e_txwqe_complete context is not good

enough since both the compiler and CPU are free to reorder these two

functions. If reordering does occur, the issue that was supposedly fixed by

7e3f3ba97e6c ("net/mlx5e: Track xmit submission to PTP WQ after populating

metadata map") will be seen. This will lead to NULL pointer dereferences in

mlx5e_ptpsq_mark_ts_cqes_undelivered in the NAPI polling context due to the

tracking list being populated before the metadata map.