md/raid6: Fix the problem of repeatedly applying for memory in raid5_read_one_chunk

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6OSXU



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commit "md/raid6: refactor raid5_read_one_chunk" incorrectly merged the
code.
Repeatedly applying for memory leads to memory leaks.

Fix it by removing redundant allocating memory code.

Fixes: c13c2cd2 ("md/raid6: refactor raid5_read_one_chunk")
Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>