lockd: pin next file across nlm_inspect_file lock-drop

Summary

In the Linux kernel, the following vulnerability has been resolved: lockd: pin next file across nlm_inspect_file lock-drop nlm_traverse_files() pins the current file with f_count++ across a mutex_unlock for nlm_inspect_file(), but nothing pins the saved next pointer. A concurrent nlm_release_file() can kfree the next file during the unlock window, and the iterator dereferences freed memory on the next loop step. Pin both current and next before the lock-drop. Advance by swapping the pinned cursors at the end of each iteration so next is always held alive across the unlock. Always call nlm_file_release() after dropping the iteration pin, regardless of whether the file matched the predicate. Use nlm_file_inuse(), which does a live walk of the inode lock list, rather than the cached f_locks field, so skipped files that never ran nlm_inspect_file() are evaluated correctly. Because every file in a hash bucket is now pinned and released, files skipped by the is_failover_file predicate that have no locks, blocks, shares, or external references are deleted during traversal. The old code never evaluated skipped files for cleanup. The new behavior is intentional: such files are stale and should not persist in the table.