perf report: Do not process non-JIT BPF ksymbol events

The length of PERF_RECORD_KSYMBOL for BPF is a size of JITed code so
it'd be 0 when it's not JITed.  The ksymbol is needed to symbolize the
code when it gets samples in the region but non-JITed code cannot get
samples.  Thus it'd be ok to ignore them.

Actually it caused a performance issue in the perf tools on old ARM
kernels where it can refuse to JIT some BPF codes.  It ended up
splitting the existing kernel map (kallsyms).  And later lookup for a
kernel symbol would create a new kernel map from kallsyms and then
split it again and again. :(

Probably there's a bug in the kernel map/symbol handling in perf tools.
But I think we need to fix this anyway.

Reported-by: Kevin Nomura <nomurak@google.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20250305232838.128692-1-namhyung@kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

tools/perf/util/machine.c

@@ -779,6 +779,10 @@ int machine__process_ksymbol(struct machine *machine __maybe_unused,
 	if (dump_trace)
 		perf_event__fprintf_ksymbol(event, stdout);
 
+	/* no need to process non-JIT BPF as it cannot get samples */
+	if (event->ksymbol.len == 0)
+		return 0;
+
 	if (event->ksymbol.flags & PERF_RECORD_KSYMBOL_FLAGS_UNREGISTER)
 		return machine__process_ksymbol_unregister(machine, event,
 							   sample);