Fix vac_update_relstats to ensure it always sends a relcache inval message,

even if none of the fields in the pg_class row change.  This behavior is
necessary to ensure other backends flush rd_targblock values that might
point to truncated-away pages.  We got this right pre-8.2 but it was broken
by overoptimistic change to not write out the pg_class row if unchanged.
Per report from Pavan Deolasee.