net/mlx5: Avoid NULL pointer dereference on steering cleanup

On cleanup, when the node is the last child of parent then it calls to
tree_put_node on the parent, if the parent's reference count
is decremented to 0 (for e.g. when deleting last destination of FTE)
then we free the parent as well and vice versa. In such a case
we will try to free the parent node again.
Increment the parent reference count before cleaning it's children
will prevent implicit release of the parent object.

Fixes: 0da2d666 ('net/mlx5: Properly remove all steering objects')
signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>