Fix bug in the new B-tree incomplete-split code.

Inserting a downlink to an internal page clears the incomplete-split flag
of the child's left sibling, so the left sibling's LSN also needs to be
updated.

src/backend/access/nbtree/nbtinsert.c

@@ -910,6 +910,10 @@ _bt_insertonpg(Relation rel,
 			{
 				PageSetLSN(metapg, recptr);
 			}
+			if (BufferIsValid(cbuf))
+			{
+				PageSetLSN(BufferGetPage(cbuf), recptr);
+			}
 
 			PageSetLSN(page, recptr);
 		}
@@ -1402,6 +1406,10 @@ _bt_split(Relation rel, Buffer buf, Buffer cbuf, OffsetNumber firstright,
 		{
 			PageSetLSN(spage, recptr);
 		}
+		if (!isleaf)
+		{
+			PageSetLSN(BufferGetPage(cbuf), recptr);
+		}
 	}
 
 	END_CRIT_SECTION();