test-loop-block: let's explicitly flush buffer cache on whole block device
    
    Let's explicitly flush the kernel's buffer cache on the whole block
    device once we ran "mkfs". This is necessary, because partition and
    whole block devices maintain separate buffer caches, and thus writing
    to one will not be visible on the other if cached there already, until
    the latter's cache is explicitly flushed.
    
    This is preparation for later adding support for probing file sytems
    also if we have no open partition block devices, and hence want to use
    the whole block device instead.
    
    (cherry picked from commit 59a4c0d7e362269c708ab8359f7e47e952ab245e)