fsperf: Workaround perf changing cwd via setns(2)

"perf report" calls setns(2) to check process details, and it may change
CWD of this process. Probably, bug of perf.

To workaround this behavior, this makes sure cwd is changing back to
original by perf_fix_cwd().

fsperf/fsperf.pl

@@ -247,6 +247,16 @@ (@)
     warn "Warning: @_\n";
 }
 
+# "perf report" calls setns(2) to check process details, and it may
+# change CWD of this process.  Make sure, change back to original here.
+sub perf_fix_cwd
+{
+    if (not $ENV{FSPERF_CWD}) {
+	$ENV{FSPERF_CWD} = getcwd();
+    }
+    chdir($ENV{FSPERF_CWD});
+}
+
 sub safe_system_with_output
 {
     my $output = shift;
@@ -387,6 +397,8 @@ ($;$$)
     my $curdir = shift;
     my $path;
 
+    perf_fix_cwd();
+
     if ($curdir) {
 	$path = $name;
     } else {
@@ -447,6 +459,7 @@ ($)
     my $re_addr16 = $re_addr . "{16}";
 
     # No way to dump kallsyms by perf, so use "strings"
+    perf_fix_cwd();
     open(my $fh, "-|", "strings $data") or die "Couldn't run `strings': $!";
 
     my $last = -1;
@@ -4867,6 +4880,8 @@ sub trace_end
     my $mode = $ENV{FSPERF_MODE};
     my $func = $mode_table{$mode}->{func};
 
+    perf_fix_cwd();
+
     # Call post process function
     $func->();
 
@@ -5577,6 +5592,8 @@ sub cmd_debug_run
     exit(0);
 }
 
+perf_fix_cwd();
+
 my %cmd_func = (
 		"record"	=> \&cmd_record,
 		"report"	=> \&cmd_report,