Fix incorrect variable datatype.

Since slot_keep_segs indicates the number of WAL segments not LSN,
its datatype should not be XLogRecPtr.

Back-patch to v13 where this issue was added.

Reported-by: Atsushi Torikoshi
Author: Atsushi Torikoshi, tweaked by Fujii Masao
Discussion: https://postgr.es/m/ebd0d674f3e050222238a960cac5251a@oss.nttdata.com

src/backend/access/transam/xlog.c

@@ -9601,7 +9601,7 @@ KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
 		/* Cap by max_slot_wal_keep_size ... */
 		if (max_slot_wal_keep_size_mb >= 0)
 		{
-			XLogRecPtr	slot_keep_segs;
+			uint64		slot_keep_segs;
 
 			slot_keep_segs =
 				ConvertToXSegs(max_slot_wal_keep_size_mb, wal_segment_size);