CVE-2026-89723: nilfs2: fix slab-out-of-bounds in nilfs_direct_propagate after truncation

In the Linux kernel, the following vulnerability has been resolved:

nilfs2: fix slab-out-of-bounds in nilfsdirectpropagate after truncation

Shuangpeng Bai reported that KASAN detected a slab-out-of-bounds error in nilfsdirectpropagate() during testing.

Analysis revealed that after truncating a file, a node block immediately below the B-tree root was not deleted. Instead, it remained in the B-tree node cache in a dirty state. The log writer subsequently detected this block and incorrectly invoked nilfsdirectpropagate() on it, which is designed to handle only data blocks in direct mapping.

B-tree nodes in the cache are managed by virtual block numbers, and their logical keys typically exceed the range expected by direct mapping. Consequently, processing such a node as a direct mapping entry triggers a slab-out-of-bounds access.

The root cause is that when a B-tree mapping collapses into a direct mapping during truncation, an intermediate node block pointed to by the root node is left behind as garbage instead of being explicitly deleted.

This resolves the issue by adding a nilfsbtreediscard() operation to delete the remaining intermediate node block during the conversion. A 'deform' flag is added to the bopdelete interface to explicitly signal that the deletion is part of a mapping transformation. This allows the B-tree mapping implementation to perform the necessary cleanup and discarding of the residual node structure that would be otherwise be left orphaned after the transition.