blk-mq: add exception handling when srcu->sda alloc failed

hulk inclusion
category: bugfix
bugzilla: 186352, https://gitee.com/openeuler/kernel/issues/I4YADX


CVE: NA

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In case of BLK_MQ_F_BLOCKING, per-hctx srcu is used to protect dispatch
critical area. But the current process is not aware when memory of srcu
allocation failed in blk_mq_alloc_hctx, which will leads to illegal
address BUG. Add return value validation to avoid this problem.

Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>