CVE-2025-38058 __legitimize_mnt(): check for MNT_SYNC_UMOUNT should be under mount_lock

In the Linux kernel, the following vulnerability has been resolved: legitimizemnt: check for MNTSYNCUMOUNT should be under mountlock ... or we risk stealing final mntput from sync umount - raising mntcount after umount2 has verified that victim is not busy, but before it has set MNTSYNCUMOUNT; in...