mkfs.gfs2: Open the target device with O_EXCL
    
    O_EXCL will let local mounters know that the device is busy while
    mkfs.gfs2 is running so that they don't try to access it.
    
    Before:
      # mount /dev/vdc /mnt/test
      mount: /dev/vdc: can't read superblock
    
    With O_EXCL:
      # mount /dev/vdc /mnt/test
      mount: /dev/vdc is already mounted or /mnt/test busy
    
    Signed-off-by: Andrew Price <anprice@redhat.com>