net/mlx5e: Introduce a fenced NOP WQE posting function

Similar to the existing mlx5e_post_nop(), but marks a fence
in the WQE control segment.

Added as a separate new function to not hurt the performance
of the common case.

To be used in a downstream patch of the series.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Boris Pismenny <borisp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>