perf intel-pt: Refine kernel decoding only warning message

perf intel-pt: Refine kernel decoding only warning message

Stop the message displaying when user space is not being traced.

Example:

  Prerequisites:

    sudo setcap "cap_sys_rawio,cap_sys_admin,cap_sys_ptrace,cap_syslog,cap_ipc_lock=ep" ~/bin/perf
    sudo chmod +r /proc/kcore

  Before:

    $ perf record --no-switch-events --kcore -a -e intel_pt//k -- sleep 0.001
    Warning:
    Intel Processor Trace decoding will not be possible except for kernel tracing!
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.838 MB perf.data ]

  After:

    $ perf record --no-switch-events --kcore -a -e intel_pt//k -- sleep 0.001
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 1.068 MB perf.data ]

    $ sudo chmod go-r /proc/kcore
    $ sudo setcap -r ~/bin/perf

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: http://lore.kernel.org/lkml/20200528120859.21604-2-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>