bpf: add __printf verification to bpf_verifier_vlog

__printf is useful to verify format and arguments. ‘bpf_verifier_vlog’
function is used twice in verifier.c in both cases the caller function
already uses the __printf gcc attribute.

Remove the following warning, triggered with W=1:

  kernel/bpf/verifier.c:176:2: warning: function might be possible candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/bpf_verifier.h

@@ -200,8 +200,8 @@ struct bpf_verifier_env {
 	u32 subprog_cnt;
 };
 
-void bpf_verifier_vlog(struct bpf_verifier_log *log, const char *fmt,
-		       va_list args);
+__printf(2, 0) void bpf_verifier_vlog(struct bpf_verifier_log *log,
+				      const char *fmt, va_list args);
 __printf(2, 3) void bpf_verifier_log_write(struct bpf_verifier_env *env,
 					   const char *fmt, ...);