filelock: SoftFileLock removes a live holder's marker when another user forges the PID in a writable marker

SoftFileLock, MarkerSoftFileLock, SoftFileLease, their async variants and a FileLock that has fallen back to a soft lock on ENOSYS decide from the record inside the marker file whether its owner is gone. The marker is the file whose presence says a process holds the lock. They do not check who...