savemeta: Don't trim off trailing zeroes when compressing
    
    The code that trims the trailing zeroes from blocks before saving them
    always features as one of the slow points in performance reports. When
    we're compressing the file anyway, the compression algos should do this
    much more efficiently than we can, so short circuit it when compression
    is enabled.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>