CVE-2024-38582: nilfs2: fix potential hang in nilfs_detach_log_writer()

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

nilfs2: fix potential hang in nilfsdetachlogwriter()

Syzbot has reported a potential hang in nilfsdetachlogwriter() called during nilfs2 unmount.

Analysis revealed that this is because nilfssegctorsync(), which synchronizes with the log writer thread, can be called after nilfssegctordestroy() terminates that thread, as shown in the call trace below:

nilfsdetachlogwriter nilfssegctordestroy nilfssegctorkillthread --> Shut down log writer thread flushwork nilfsiputworkfunc nilfsdisposelist iput nilfsevictinode nilfstransactioncommit nilfsconstructsegment (if inode needs sync) nilfssegctorsync --> Attempt to synchronize with log writer thread DEADLOCK

Fix this issue by changing nilfssegctorsync() so that the log writer thread returns normally without synchronizing after it terminates, and by forcing tasks that are already waiting to complete once after the thread terminates.

The skipped inode metadata flushout will then be processed together in the subsequent cleanup work in nilfssegctordestroy().