Fix function code in error report

This bug causes a lseek() failure to be reported as a "could not open"
failure in the error message, muddling bug reports.  I introduced this
copy-and-pasteo in commit 78e122010422.

Noticed while reviewing code for bug report #15221, from lily liang.  In
version 10 the affected function is only used by multixact.c and
commit_ts, and only in corner-case circumstances, neither of which are
involved in the reported bug (a pg_subtrans failure.)

Author: Álvaro Herrera

src/backend/access/transam/slru.c

@@ -614,7 +614,7 @@ SimpleLruDoesPhysicalPageExist(SlruCtl ctl, int pageno)
 
 	if ((endpos = lseek(fd, 0, SEEK_END)) < 0)
 	{
-		slru_errcause = SLRU_OPEN_FAILED;
+		slru_errcause = SLRU_SEEK_FAILED;
 		slru_errno = errno;
 		SlruReportIOError(ctl, pageno, 0);
 	}