Simplify some maths in xlogreader.c

An LSN was calculated from a segment number, a segment size and a
position offset, matching exactly the LSN given by the caller of
XLogReaderValidatePageHeader().  This change removes the extra LSN
calculation, relying only on the LSN given by the function caller
instead.

Author: Bharath Rupireddy
Reviewed-by: Richard Guo, Álvaro Herrera, Kyotaro Horiguchi
Discussion: https://postgr.es/m/CALj2ACXuh4Ms9j9sxMYdtHEe=5sFcyrs-GAHyADu_A_G71kZTg@mail.gmail.com

Author: michaelpq

src/backend/access/transam/xlogreader.c

@@ -1210,7 +1210,6 @@ bool
 XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,
 							 char *phdr)
 {
-	XLogRecPtr	recaddr;
 	XLogSegNo	segno;
 	int32		offset;
 	XLogPageHeader hdr = (XLogPageHeader) phdr;
@@ -1220,8 +1219,6 @@ XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,
 	XLByteToSeg(recptr, segno, state->segcxt.ws_segsize);
 	offset = XLogSegmentOffset(recptr, state->segcxt.ws_segsize);
 
-	XLogSegNoOffsetToRecPtr(segno, offset, state->segcxt.ws_segsize, recaddr);
-
 	if (hdr->xlp_magic != XLOG_PAGE_MAGIC)
 	{
 		char		fname[MAXFNAMELEN];
@@ -1296,7 +1293,7 @@ XLogReaderValidatePageHeader(XLogReaderState *state, XLogRecPtr recptr,
 	 * check typically fails when an old WAL segment is recycled, and hasn't
 	 * yet been overwritten with new data yet.
 	 */
-	if (hdr->xlp_pageaddr != recaddr)
+	if (hdr->xlp_pageaddr != recptr)
 	{
 		char		fname[MAXFNAMELEN];