net/mlx5e: Do not modify the TX SKB

If the SKB is cloned, or has an elevated users count, someone else
can be looking at it at the same time.

Signed-off-by: Achiad Shochat <achiad@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>