net/mlx5: fs, make get_root_namespace API function

As preparation for HW Steering support, where the function
get_root_namespace() is needed to get root FDB, make it an API function
and rename it to mlx5_get_root_namespace().

Reviewed-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Signed-off-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Link: https://patch.msgid.link/20240911201757.1505453-5-saeed@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>