mkfs: Handle gfs2 creation on regular files
    
    mkfs.gfs2 previously exited if the target volume was a regular file.
    This patch allows creation of gfs2 file systems on regular files by
    removing an exiting check, guarding against calling device_topology()
    and falling back to the default block size if the target is a regular
    file. It also removes the check_mount() function and replaces it with an
    fstat() call after opening the file to avoid races.
    
    rhbz#720668
    
    Signed-off-by: Andrew Price <anprice@redhat.com>
    Reviewed-by: Steven Whitehouse <swhiteho@redhat.com>