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

euler inclusion
category: bugfix
bugzilla: 15743
CVE: NA
---------------------------

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>
Reviewed-by: Miao Xie <miaoxie@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>