bpf: make function skip_callee static and return NULL rather than 0

Function skip_callee is local to the source and does not need to
be in global scope, so make it static. Also return NULL rather than 0.
Cleans up two sparse warnings:

symbol 'skip_callee' was not declared. Should it be static?
Using plain integer as NULL pointer
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

kernel/bpf/verifier.c

@@ -823,6 +823,7 @@ static int check_subprogs(struct bpf_verifier_env *env)
 	return 0;
 }
 
+static
 struct bpf_verifier_state *skip_callee(struct bpf_verifier_env *env,
 				       const struct bpf_verifier_state *state,
 				       struct bpf_verifier_state *parent,
@@ -867,7 +868,7 @@ struct bpf_verifier_state *skip_callee(struct bpf_verifier_env *env,
 	verbose(env, "verifier bug regno %d tmp %p\n", regno, tmp);
 	verbose(env, "regno %d parent frame %d current frame %d\n",
 		regno, parent->curframe, state->curframe);
-	return 0;
+	return NULL;
 }
 
 static int mark_reg_read(struct bpf_verifier_env *env,