gve: Fix error return code in gve_prefill_rx_pages()

If alloc_page() fails in gve_prefill_rx_pages(), it should return
an error code in the error path.

Fixes: 82fd151d ("gve: Reduce alloc and copy costs in the GQ rx path")
Cc: Jeroen de Borst <jeroendb@google.com>
Cc: Catherine Sullivan <csully@google.com>
Cc: Shailend Chand <shailend@google.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>