perf trace: Beautify sched_setscheduler 'policy' argument

perf trace: Beautify sched_setscheduler 'policy' argument

  $ trace -e sched_setscheduler chrt -f 1 usleep 1
  chrt: failed to set pid 0's policy: Operation not permitted
     0.005 ( 0.005 ms): chrt/19189 sched_setscheduler(policy: FIFO, param: 0x7ffec5273d70) = -1 EPERM Operation not permitted
  $

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-i5vlo5n5jv0amt8bkyicmdxh@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>