fs/dcache.c: avoid softlock since too many negative dentry

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8JH5M?from=project-issue



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Parallel thread to add negative dentry under root dir. Sometimes later,
'systemctl daemon-reload' will report softlockup since
__fsnotify_update_child_dentry_flags need update all child under root
dentry without distinguish does it active or not. It will waste so long
time with catching d_lock of root dentry. And other thread try to
spin_lock d_lock will run overtime.

Limit negative dentry under dir can avoid this.

Signed-off-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Zizhi Wo <wozizhi@huawei.com>