bpfilter: Specify the log level for the kmsg message

Per the kmsg document [0], if we don't specify the log level with a
prefix "<N>" in the message string, the default log level will be
applied to the message. Since the default level could be warning(4),
this would make the log utility such as journalctl treat the message,
"Started bpfilter", as a warning. To avoid confusion, this commit
adds the prefix "<5>" to make the message always a notice.

  [0] https://www.kernel.org/doc/Documentation/ABI/testing/dev-kmsg

Fixes: 36c4357 ("net: bpfilter: print umh messages to /dev/kmsg")
Reported-by: Martin Loviska <[email protected]>
Signed-off-by: Gary Lin <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Dmitrii Banshchikov <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: lcp

net/bpfilter/main.c

@@ -57,7 +57,7 @@ int main(void)
 {
 	debug_f = fopen("/dev/kmsg", "w");
 	setvbuf(debug_f, 0, _IOLBF, 0);
-	fprintf(debug_f, "Started bpfilter\n");
+	fprintf(debug_f, "<5>Started bpfilter\n");
 	loop();
 	fclose(debug_f);
 	return 0;