mkfs.gfs2: Free unnecessary cached pages, disable readahead
    
    With a 300T file system, cached pages built up quite heavily and could
    be left over from previous io on the device. Use POSIX_FADV_DONTNEED
    where appropriate.
    
    Also, readahead isn't required as mkfs.gfs2 mainly writing full block
    ranges at unpredictable locations so use POSIX_FADV_RANDOM to minimise
    the overhead from that.
    
    Resolves: rhbz#1440269
    
    Signed-off-by: Andrew Price <anprice@redhat.com>