net/mlx5: Allocate root ns memory using kzalloc to match kfree

root ns is yet another fs core node which is freed using kfree() by
tree_put_node().
Rest of the other fs core objects are also allocated using kmalloc
variants.

However, root ns memory is allocated using kvzalloc().
Hence allocate root ns memory using kzalloc().

Fixes: 25302363 ("net/mlx5_core: Flow steering tree initialization")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>