pgbench: Increase maximum size of log filename from 64 to MAXPGPATH.

Commit 41124a91 allowed the
transaction log file prefix to be changed but left in place the
existing 64-character limit on the total length of a log file name.
It's possible that could be inconvenient for somebody, so increase the
limit to MAXPGPATH, which ought to be enough for anybody.

Per a suggestion from Tom Lane.

src/bin/pgbench/pgbench.c

@@ -4401,7 +4401,7 @@ threadRun(void *arg)
 	/* open log file if requested */
 	if (use_log)
 	{
-		char		logpath[64];
+		char		logpath[MAXPGPATH];
 		char		*prefix = logfile_prefix ? logfile_prefix : "pgbench_log";
 
 		if (thread->tid == 0)