RDMA/hns: Fix unregistering device and accessing to debugfs concurrently

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



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When unregistering the device or destroying of ucontext and accessing
dca debugfs concurrently, there may be a problem of accessing a null
pointer.

This patch fixes it by delaying the pointer assignment to null until
debugfs has been unregistered.

Fixes: a2178118 ("RDMA/hns: Add debugfs support for DCA")
Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>