perf top: Lower message level for failure on synthesizing events for pre-existing BPF programs

Move it from being a pr_warning() to a pr_debug(). Also capitalize BPF
and explain what gets missing when we're not able to synthesize these
events: we'll not be able to resolve symbols, etc.

Reported-by: Ingo Molnar <mingo@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Yonghong Song <yhs@fb.com>
Link: https://lkml.kernel.org/n/tip-whpnfnw6xtd939odgt9bw9as@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-top.c

@@ -1215,7 +1215,7 @@ static int __cmd_top(struct perf_top *top)
 						&top->session->machines.host,
 						&top->record_opts);
 	if (ret < 0)
-		pr_warning("Couldn't synthesize bpf events.\n");
+		pr_debug("Couldn't synthesize BPF events: Pre-existing BPF programs won't have symbols resolved.\n");
 
 	machine__synthesize_threads(&top->session->machines.host, &opts->target,
 				    top->evlist->threads, false,