landlock: Clean up interrupted thread logic in TSYNC

landlock: Clean up interrupted thread logic in TSYNC

In landlock_restrict_sibling_threads(), when the calling thread is
interrupted while waiting for sibling threads to prepare, it executes
a recovery path.

Previously, this path included a wait_for_completion() call on
all_prepared to prevent a Use-After-Free of the local shared_ctx.
However, this wait is redundant. Exiting the main do-while loop
already leads to a bottom cleanup section that unconditionally waits
for all_finished. Therefore, replacing the wait with a simple break
is safe, prevents UAF, and correctly unblocks the remaining task_works.

Clean up the error path by breaking the loop and updating the
surrounding comments to accurately reflect the state machine.

Suggested-by: Günther Noack <gnoack3000@gmail.com>
Signed-off-by: Yihan Ding <dingyihan@uniontech.com>
Tested-by: Günther Noack <gnoack3000@gmail.com>
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260306021651.744723-3-dingyihan@uniontech.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>