If an error occurs in the process after the SGL is mapped
successfully, it need to unmap the SGL.

Otherwise, memory problems may occur.
Signed-off-by: Yang Shen <shenyang39@huawei.com>
Signed-off-by: Chenghai Huang <huangchenghai2@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>