bpf: Replace hard-coded 0 with BPF_K in check_alu_op

Enhance readability a bit.
Signed-off-by: Simon Wang <wangchuanguo@inspur.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20220622031923.65692-1-wangchuanguo@inspur.com

kernel/bpf/verifier.c

@@ -9096,7 +9096,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
 
 	if (opcode == BPF_END || opcode == BPF_NEG) {
 		if (opcode == BPF_NEG) {
-			if (BPF_SRC(insn->code) != 0 ||
+			if (BPF_SRC(insn->code) != BPF_K ||
 			    insn->src_reg != BPF_REG_0 ||
 			    insn->off != 0 || insn->imm != 0) {
 				verbose(env, "BPF_NEG uses reserved fields\n");