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>

Author: Al Viro

fs/libfs.c

@@ -613,7 +613,7 @@ void simple_recursive_removal(struct dentry *dentry,
 		struct dentry *victim = NULL, *child;
 		struct inode *inode = this->d_inode;
 
-		inode_lock(inode);
+		inode_lock_nested(inode, I_MUTEX_CHILD);
 		if (d_is_dir(this))
 			inode->i_flags |= S_DEAD;
 		while ((child = find_next_child(this, victim)) == NULL) {
@@ -625,7 +625,7 @@ void simple_recursive_removal(struct dentry *dentry,
 			victim = this;
 			this = this->d_parent;
 			inode = this->d_inode;
-			inode_lock(inode);
+			inode_lock_nested(inode, I_MUTEX_CHILD);
 			if (simple_positive(victim)) {
 				d_invalidate(victim);	// avoid lost mounts
 				if (callback)