RDMA/hns: Improve the readability of free mr exception branch

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



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The code of the current free mr abnormal branch is not readable,
resulting in false positives when checked by tools such as smatch.

This patch combines the processing of exception branches into a
separate function to improve readability.

Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>