bpf: refactor "check_reg_arg" to eliminate code redundancy

There are a few "regs[regno]" here are there across "check_reg_arg", this
patch factor it out into a simple "reg" pointer. The intention is to
simplify code indentation and make the later patches in this set look
cleaner.
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -1177,30 +1177,32 @@ static int check_reg_arg(struct bpf_verifier_env *env, u32 regno,
 {
 	struct bpf_verifier_state *vstate = env->cur_state;
 	struct bpf_func_state *state = vstate->frame[vstate->curframe];
-	struct bpf_reg_state *regs = state->regs;
+	struct bpf_reg_state *reg, *regs = state->regs;
 
 	if (regno >= MAX_BPF_REG) {
 		verbose(env, "R%d is invalid\n", regno);
 		return -EINVAL;
 	}
 
+	reg = &regs[regno];
 	if (t == SRC_OP) {
 		/* check whether register used as source operand can be read */
-		if (regs[regno].type == NOT_INIT) {
+		if (reg->type == NOT_INIT) {
 			verbose(env, "R%d !read_ok\n", regno);
 			return -EACCES;
 		}
 		/* We don't need to worry about FP liveness because it's read-only */
-		if (regno != BPF_REG_FP)
-			return mark_reg_read(env, &regs[regno],
-					     regs[regno].parent);
+		if (regno == BPF_REG_FP)
+			return 0;
+
+		return mark_reg_read(env, reg, reg->parent);
 	} else {
 		/* check whether register used as dest operand can be written to */
 		if (regno == BPF_REG_FP) {
 			verbose(env, "frame pointer is read only\n");
 			return -EACCES;
 		}
-		regs[regno].live |= REG_LIVE_WRITTEN;
+		reg->live |= REG_LIVE_WRITTEN;
 		if (t == DST_OP)
 			mark_reg_unknown(env, regs, regno);
 	}