lockdev: fix device locking other tools
    
    Module locks.c is compiled in not only into mgetty itself but to other
    tools too, e.g vgetty.  Initial version of patch adding support for
    lockdev api covered device locking only for mgetty. At the time
    neither me nor people who tested the patch discovered the bug,
    because obviosly we didn't tested it with voice modems.
    
    Resolves: #982711