Fix masking of SP-GiST pages during xlog consistency check

spg_mask() didn't take into account that pd_lower equal to SizeOfPageHeaderData
is still valid value.  This commit fixes that.  Backpatch to 11, where
spg_mask() pg_lower check was introduced.

Reported-by: Michael Paquier
Discussion: https://postgr.es/m/20200615131405.GM52676%40paquier.xyz
Backpatch-through: 11

Author: akorotkov

src/backend/access/spgist/spgxlog.c

@@ -1008,6 +1008,6 @@ spg_mask(char *pagedata, BlockNumber blkno)
 	 * Mask the unused space, but only if the page's pd_lower appears to have
 	 * been set correctly.
 	 */
-	if (pagehdr->pd_lower > SizeOfPageHeaderData)
+	if (pagehdr->pd_lower >= SizeOfPageHeaderData)
 		mask_unused_space(page);
 }