dissect: fix fsck
    
    Since f7725647bb41c3398a867f139efe526efe8aa1b3 when dissecting a disk
    image we operate with fds to the device nodes in question wherever we
    can. This includes when we fork off fsck, where we pass a /proc/self/fd/
    path as argument. This only works if we keep that fd open however and
    disable O_CLOEXEC on the fd. Hence do so, and fix fsck this way.
    
    (Without this, all fsck will fail, since the fd path is invalid)
    
    (cherry picked from commit f8ab781223bcb0330ee4499b879a62e84fee313e)