net/mlx5: Refactor and optimize flow counter bulk query

Towards introducing the ability to allocate bulks of flow counters,
refactor the flow counter bulk query process, removing functions and
structs whose names indicated being used for flow counter bulk
allocation FW commands, despite them actually only being used to
support bulk querying, and migrate their functionality to correctly
named functions in their natural location, fs_counters.c.

Additionally, optimize the bulk query process by:
 * Extracting the memory used for the query to mlx5_fc_stats so
   that it is only allocated once, and not for each bulk query.
 * Querying all the counters in one function call.

Signed-off-by: Gavi Teitz <gavi@mellanox.com>
Reviewed-by: Vlad Buslov <vladbu@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>