perf/trace/scripting: failed-syscalls script cleanup

A couple small fixes for the failed syscalls script:

- The script description says it can be restricted to a specific comm,
make it so.

- silence the match output in the shell script

Cc: Frédéric Weisbecker <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
LKML-Reference: <[e-mail]>
Signed-off-by: Tom Zanussi <[e-mail]>
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/scripts/perl/bin/failed-syscalls-report

···
 # description: system-wide failed syscalls
 # args: [comm]
 if [ $# -gt 0 ] ; then
-    if ! expr match "$1" "-" ; then
+    if ! expr match "$1" "-" > /dev/null ; then
 	comm=$1
 	shift
     fi

tools/perf/scripts/perl/failed-syscalls.pl

···
 use Perf::Trace::Context;
 use Perf::Trace::Util;
 
+my $for_comm = shift;
+
 my %failed_syscalls;
 
 sub raw_syscalls::sys_exit
···
 
     foreach my $comm (sort {$failed_syscalls{$b} <=> $failed_syscalls{$a}}
 		      keys %failed_syscalls) {
-	    printf("%-20s  %10s\n", $comm, $failed_syscalls{$comm});
+	next if ($for_comm && $comm ne $for_comm);
+
+	printf("%-20s  %10s\n", $comm, $failed_syscalls{$comm});
     }
 }