net/mlx5: Add new list to store deleted flow counters

In order to prevent flow counters stats work function from traversing whole
flow counters tree while searching for deleted flow counters, new list to
store deleted flow counters is added to struct mlx5_fc_stats. Lockless
NULL-terminated single linked list data type is used due to following
reasons:
 - This use case only needs to add single element to list and
 remove/iterate whole list. Lockless list doesn't require any additional
 synchronization for these operations.
 - First cache line of flow counter data structure only has space to store
 single additional pointer, which precludes usage of double linked list.

Remove flow counter 'deleted' flag that is no longer needed.

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Acked-by: Amir Vadai <amir@vadai.me>
Reviewed-by: Paul Blakey <paulb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>