If statvfs does not exist, then don't recalculate mount flags
at remount.

If someone does need this, they could replace the code (only
if !HAVE_STATVFS) with code parsing /proc/self/mountinfo (which
exists in the recent git history)
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>