RDMA/hns: Remove unnecessary checks for NULL in mtr_alloc_bufs()

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



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ib_umem_get() never return NULL.

Fixes: 3c873161 ("RDMA/hns: Add support for addressing when hopnum is 0")
Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>