perf help: Elliminate dup code for reporting

And switch from warning() to pr_warning(), to elliminate another
duplication: too many error reporting facilities.

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-pkzcjrhek3uuqc4i5i9ealwd@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>