RDMA/hns: Fix potential NULL pointer in DCA memory query

driver inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I87LCF



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The hr_qp can be a NULL pointer. A check has been added to avoid
illegal access.

Fixes: f0384ddc ("RDMA/hns: Add method to query WQE buffer's address")
Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>