Rename variable for code clarity

When tracking IO timing for WAL, the duration is what we calculate
based on the start and end timestamps, it's not what the variable
contains. Rename the timestamp variable to end to better communicate
what it contains.  Original patch by Krishnakumar with additional
hacking to fix another occurrence by me.

Author: Krishnakumar R <kksrcv001@gmail.com>
Discussion: https://postgr.es/m/CAPMWgZ9f9o8awrQpjo8oxnNQ=bMDVPx00NE0QcDzvHD_ZrdLPw@mail.gmail.com

src/backend/access/transam/xlog.c

@@ -2226,10 +2226,10 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 				 */
 				if (track_wal_io_timing)
 				{
-					instr_time	duration;
+					instr_time	end;
 
-					INSTR_TIME_SET_CURRENT(duration);
-					INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_write_time, duration, start);
+					INSTR_TIME_SET_CURRENT(end);
+					INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_write_time, end, start);
 				}
 
 				PendingWalStats.wal_write++;
@@ -8252,10 +8252,10 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 	 */
 	if (track_wal_io_timing)
 	{
-		instr_time	duration;
+		instr_time	end;
 
-		INSTR_TIME_SET_CURRENT(duration);
-		INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_sync_time, duration, start);
+		INSTR_TIME_SET_CURRENT(end);
+		INSTR_TIME_ACCUM_DIFF(PendingWalStats.wal_sync_time, end, start);
 	}
 
 	PendingWalStats.wal_sync++;