net: hns3: fix the bug when map buffer fail

If one buffer had been recieved to stack, driver will alloc a new buffer,
map the buffer to device and replace the old buffer. When map fail, should
only free the new alloced buffer, but not free all buffers in the ring.

Fixes: 76ad4f0e (net: hns3: Add support of HNS3 Ethernet Driver for hip08 SoC)

Signed-off-by: Lipeng <lipeng321@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>