Fix variable confusion in BufferSync().

Fix variable confusion in BufferSync().

As noted by Heikki Linnakangas, the previous coding confused the "flags"
variable with the "mask" variable.  The affect of this appears to be that
unlogged buffers would get written out at every checkpoint rather than
only at shutdown time.  Although that's arguably an acceptable failure
mode, I'm back-patching this change, since it seems like a poor idea to
rely on this happening to work.