Back-patch 8.2 fix that complains if trying to extend a relation encounters

Back-patch 8.2 fix that complains if trying to extend a relation encounters
a buffer containing a non-zeroed page.  This seems appropriate now that the
8.2 fix has been seen to save at least one user from data loss due to a
buggy kernel (per report from Jaime Silvela 7-May-07).  I'd go further
back than 8.1, except that the 8.0-to-8.1 bufmgr changes are large
enough that the patch doesn't work immediately; I'm hesitant to make a
change without more extensive analysis than I have time for now.