Previously, when changing the ring parameters, we free the old
ring resources firstly, and then setup the new ring resources.
In some case of an memory allocation fail, there will be no
resources to use. This patch refines it by setup new ring
resources and free the old ring resources in order.

Also reduce the max ring BD number to 32760 according to UM.
Signed-off-by: Jian Shen <shenjian15@huawei.com>
Reviewed-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>