configfs: factor dirent removal into helpers

mainline inclusion
from mainline-v5.4-rc1
commit 1cf7a003
category: bugfix
bugzilla: 187567, https://gitee.com/openeuler/kernel/issues/I5PK1G


CVE: NA

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Lots of duplicated code that benefits from a little consolidation.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>