perf trace: Beautify pipe2 'flags' arg

 4.234 (0.005 ms): fetchmail/3224 pipe2(fildes: 0x7fffc72bcee0, flags: CLOEXEC) = 0

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-9e1jz78i6q6e0xr9fsitqbpe@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>