net/mlx5: Don't use already freed action pointer

The call to mlx5dr_action_destroy() releases "action" memory. That
pointer is set to miss_action later and generates the following smatch
error:

 drivers/net/ethernet/mellanox/mlx5/core/steering/fs_dr.c:53 set_miss_action()
 warn: 'action' was already freed.

Make sure that the pointer is always valid by setting NULL after destroy.

Fixes: 6a48faee

 ("net/mlx5: Add direct rule fs_cmd implementation")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>