CVE-2025-21721: nilfs2: handle errors that nilfs_prepare_chunk() may return

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

nilfs2: handle errors that nilfspreparechunk() may return

Patch series "nilfs2: fix issues with rename operations".

This series fixes BUGON check failures reported by syzbot around rename operations, and a minor behavioral issue where the mtime of a child directory changes when it is renamed instead of moved.

This patch (of 2):

The directory manipulation routines nilfssetlink() and nilfsdeleteentry() rewrite the directory entry in the folio/page previously read by nilfsfindentry(), so error handling is omitted on the assumption that nilfspreparechunk(), which prepares the buffer for rewriting, will always succeed for these. And if an error is returned, it triggers the legacy BUGON() checks in each routine.

This assumption is wrong, as proven by syzbot: the buffer layer called by nilfspreparechunk() may call nilfsgetblock() if necessary, which may fail due to metadata corruption or other reasons. This has been there all along, but improved sanity checks and error handling may have made it more reproducible in fuzzing tests.

Fix this issue by adding missing error paths in nilfssetlink(), nilfsdeleteentry(), and their caller nilfsrename().