shutdown: in the final umount loop don't use MNT_FORCE

MNT_FORCE is honoured by NFS and FUSE and allows unmounting of the FS
even if consumers still use it. For our brute-force loop we rely on
EBUSY being reported as long as a file system is still used by a
loopback device or suchlike. Hence, drop MNT_FORCE to make EBUSY
reliable.