ANDROID: incfs: vfs: Annotate stacked filesystem re-entry with lockdep subclasses

Lockdep reports recursive locking warnings in `incfs_setattr` and
`iterate_dir`. Because IncFS is a stacked filesystem, these operations
re-enter VFS on the backing (lower) directory or file, calling
VFS helpers or locking the lower inode directly.

Since VFS acquires the upper inode's `i_rwsem` with subclass 0, the
unannotated re-entry into the lower inode (which also uses subclass 0)
falsely triggers lockdep's recursive lock detection.

Fix this by annotating the lower inode acquisitions with the
`I_MUTEX_PARENT` subclass:
1. In `incfs_setattr`, use `inode_lock_nested(..., I_MUTEX_PARENT)`
   consistently with all other directory operations in this file.
2. In `iterate_incfs_dir`, temporarily set the backing inode's
   lock class subclass to `I_MUTEX_PARENT` around the `iterate_dir()`
   VFS helper call, and restore it to 0 immediately afterwards.

Bug: 419203261
Bug: 419264069
Change-Id: I0447aa79f91576b67290eadf143acfb87d8cb9b4
Signed-off-by: Howard Chen <howardsoc@google.com>

fs/incfs/vfs.c

@@ -384,7 +384,9 @@ static int iterate_incfs_dir(struct file *file, struct dir_context *ctx)
 	}
 
 	ctx->pos -= PSEUDO_FILE_COUNT;
+	lockdep_set_subclass(&dir->backing_dir->f_inode->i_rwsem, I_MUTEX_PARENT);
 	error = iterate_dir(dir->backing_dir, ctx);
+	lockdep_set_subclass(&dir->backing_dir->f_inode->i_rwsem, 0);
 	ctx->pos += PSEUDO_FILE_COUNT;
 	file->f_pos = dir->backing_dir->f_pos;
 out:
@@ -1621,7 +1623,7 @@ static int incfs_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 		ia->ia_mode |= 0222;
 	}
 
-	inode_lock(d_inode(backing_dentry));
+	inode_lock_nested(d_inode(backing_dentry), I_MUTEX_PARENT);
 	error = notify_change(idmap, backing_dentry, ia, NULL);
 	inode_unlock(d_inode(backing_dentry));