better lockdep annotations for simple_recursive_removal()

We want a class that nests outside of I_MUTEX_NORMAL (for the sake of
callbacks that might want to lock the victim) and inside I_MUTEX_PARENT
(so that a variant of that could be used with parent of the victim
held locked by the caller).

In reality, simple_recursive_removal()
* never holds two locks at once
* holds the lock on parent of dentry passed to callback
* is used only on the trees with fixed topology, so the depths
are not changing.

So the locking order is actually fine.

AFAICS, the best solution is to assign I_MUTEX_CHILD to the locks
grabbed by that thing.

Reported-by: syzbot+169de184e9defe7fe709@syzkaller.appspotmail.com
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>