Fix page modification outside of critical section in GIN

By oversight 52ac6cd2 makes ginDeletePage() sets pd_prune_xid of page to be
deleted before entering the critical section.  It appears that only versions 11
and later were affected by this oversight.

Backpatch-through: 11

src/backend/access/gin/ginvacuum.c

@@ -153,9 +153,6 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
 	page = BufferGetPage(dBuffer);
 	rightlink = GinPageGetOpaque(page)->rightlink;
 
-	/* For deleted page remember last xid which could knew its address */
-	GinPageSetDeleteXid(page, ReadNewTransactionId());
-
 	/*
 	 * Any insert which would have gone on the leaf block will now go to its
 	 * right sibling.
@@ -168,6 +165,9 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
 	page = BufferGetPage(lBuffer);
 	GinPageGetOpaque(page)->rightlink = rightlink;
 
+	/* For deleted page remember last xid which could knew its address */
+	GinPageSetDeleteXid(page, ReadNewTransactionId());
+
 	/* Delete downlink from parent */
 	parentPage = BufferGetPage(pBuffer);
 #ifdef USE_ASSERT_CHECKING