net: hns3: fix a bug when alloc new buffer

When alloce new buffer to HW, should unmap the old buffer first.
This old code map the old buffer but not unmap the old buffer,
this patch fixes it.

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>