perf trace: Tell arg formatters the arg index

... so that it can mask args relative to its position, like the 'mode' arg
that may or not be printed according to the 'flags' (O_CREAT) value.

 [root@zoo ~]# perf trace -a -e openat,open_by_handle_at | head -1
    469.754 ( 0.034 ms): 1183 openat(dfd: -100, filename: 0x7fbde40014b0, flags: CLOEXEC|DIRECTORY|NONBLOCK) = 23
 [root@zoo ~]#

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-bgokqpkufd4sio7ixxknf1ux@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>