RDMA/hns: Fix unhandled error code in dca uverbs

driver inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6GT7F



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uverbs_attr_get_uobject() may return a error code. The pointer is
illegal in this scenario.

Fixes: d8cca476 ("RDMA/hns: Add method for attaching WQE buffer")
Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>