eth: mlx4: Fix IS_ERR() vs NULL check bug in mlx4_en_create_rx_ring

In the Linux kernel, the following vulnerability has been resolved: eth: mlx4: Fix IS_ERR() vs NULL check bug in mlx4_en_create_rx_ring Replace NULL check with IS_ERR() check after calling page_pool_create() since this function returns error pointers (ERR_PTR). Using NULL check could lead to invalid pointer dereference.