net: hns3: refactor out hclge_add_fd_entry()

The process of function hclge_add_fd_entry() is complex and
prolix. To make it more readable, extract the process of
fs->ring_cookie to a single function.

Signed-off-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>