net/mlx5: free mlx5_st_idx_data on final dealloc

In the Linux kernel, the following vulnerability has been resolved: 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.