debugfs: fix use-after-free on symlink traversal

mainline inclusion
from mainline-5.1-rc4
commit 93b919da
category: bugfix
bugzilla: 13537
CVE: NA

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symlink body shouldn't be freed without an RCU delay.  Switch debugfs to
->destroy_inode() and use of call_rcu(); free both the inode and symlink
body in the callback.  Similar to solution for bpf, only here it's even
more obvious that ->evict_inode() can be dropped.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Reviewed-by: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>