util: Fix fallocate stubs for mingw build
    
    When any of the functions modified in commit 214c687b took false branch,
    the function itself used none of its parameters resulting in "unused
    parameter" error.  Rewriting these functions to the stubs we use
    elsewhere should fix the problem.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>