mkfs.gfs2: Scale down journal size for smaller devices
    
    Currently the default behaviour when the journal size is not specified
    is to use a default size of 128M, which means that mkfs.gfs2 can run out
    of space while writing to a small device. The hard default also means
    that some xfstests fail with gfs2 as they try to create small file
    systems.
    
    This patch addresses these problems by setting sensible default journal
    sizes depending on the size of the file system. Journal sizes specified
    by the user are limited to half of the fs. As the minimum journal size
    is 8MB that means we effectively get a hard minimum file system size of
    16MB (per journal).
    
    Resolves: rhbz#1498068
    
    Signed-off-by: Andrew Price <anprice@redhat.com>