bpf, verifier: annotate verbose printer with __printf

The verbose() printer dumps the verifier state to user space, so let gcc
take care to check calls to verbose() for (future) errors. make with W=1
correctly suggests: function might be possible candidate for 'gnu_printf'
format attribute [-Wsuggest-attribute=format].
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/verifier.c

@@ -214,7 +214,7 @@ static DEFINE_MUTEX(bpf_verifier_lock);
  * verbose() is used to dump the verification trace to the log, so the user
  * can figure out what's wrong with the program
  */
-static void verbose(const char *fmt, ...)
+static __printf(1, 2) void verbose(const char *fmt, ...)
 {
 	va_list args;