RDMA/mlx5: Refactor optional counters steering code

Currently there isn't a clear layer separation between the counters and
the steering code, whereas the steering code is doing redundant access
to the counter struct.

Separate the fs.c and counters.c, where fs code won't access or be
aware of counter structs but only the objects it needs.

As a result, move mlx5_rdma_counter struct from the header file to be
an internal struct for the counters file only.

Signed-off-by: Patrisious Haddad <phaddad@nvidia.com>
Reviewed-by: Edward Srouji <edwards@nvidia.com>
Link: https://patch.msgid.link/9854d1fdb140e4a6552b7a2fd1a028cfe488a935.1753004208.git.leon@kernel.org


Signed-off-by: Leon Romanovsky <leon@kernel.org>