net/mlx5: free mlx5_st_idx_data on final dealloc

net/mlx5: free mlx5_st_idx_data on final dealloc

Workloads that repeatedly allocate and release mkeys carrying TPH
steering-tag hints (e.g. churning RDMA MRs) leak one
struct mlx5_st_idx_data per cycle; kmemleak flags it as unreferenced
and the kmalloc slab grows over time.

When the last reference to an ST table entry is dropped,
mlx5_st_dealloc_index() removed the entry from idx_xa but the backing
mlx5_st_idx_data allocation was never freed.

Free idx_data after the xa_erase() so the lifetime of the bookkeeping
struct matches the lifetime of the ST entry it tracks.

Cc: stable@vger.kernel.org
Fixes: 888a7776f4fb ("net/mlx5: Add support for device steering tag")
Reviewed-by: Michael Gur <michaelgur@nvidia.com>
Signed-off-by: Zhiping Zhang <zhipingz@meta.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20260702222507.1234467-1-zhipingz@meta.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>