sysinfo: add Flags to Statfs_t if not already there

    
    If there is no f_flags field in statfs_t then rename one of the
    f_spare fields, as happened in Linux kernel version 2.6.36.  This
    fixes the build on CentOS 5.11.  The CentOS kernel will hopefully not
    fill in the f_spare field, so the resulting flags will be zero.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/165417

From-SVN: r269401