rx unused desc is the desc that need attatching new buffer
before refilling to hw to receive new packet, the number of
desc need attatching new buffer is calculated using next_to_use
and next_to_clean. when next_to_use == next_to_clean, currently
hns3 driver assumes that all the desc has the buffer attatched,
but 'next_to_use == next_to_clean' also means all the desc need
attatching new buffer if hw has comsumed all the desc and the
driver has not attatched any buffer to the desc yet.

This patch adds 'refill' in desc_cb to indicate whether a new
buffer has been refilled to a desc.

Fixes: 76ad4f0ee747 ("net: hns3: Add support of HNS3 Ethernet Driver for hip08 SoC")
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>