Convert sole user of warning() in this file to pr_warning(),
consolidating 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-3y7yf6v673ujl2rcs34tzv8n@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>