LoongArch: BPF: Fix sign-extension mov instructions

We can see that "Short form of movsx, dst_reg = (s8,s16,s32)src_reg" in
include/linux/filter.h, additionally, for BPF_ALU64 the value of the
destination register is unchanged whereas for BPF_ALU the upper 32 bits
of the destination register are zeroed, so it should clear the upper 32
bits for BPF_ALU.

[root@linux fedora]# echo 1 > /proc/sys/net/core/bpf_jit_enable
[root@linux fedora]# modprobe test_bpf

Before:
test_bpf: #81 ALU_MOVSX | BPF_B jited:1 ret 2 != 1 (0x2 != 0x1)FAIL (1 times)
test_bpf: #82 ALU_MOVSX | BPF_H jited:1 ret 2 != 1 (0x2 != 0x1)FAIL (1 times)

After:
test_bpf: #81 ALU_MOVSX | BPF_B jited:1 6 PASS
test_bpf: #82 ALU_MOVSX | BPF_H jited:1 6 PASS

By the way, the bpf selftest case "./test_progs -t verifier_movsx" can
also be fixed with this patch.

Fixes: f48012f1 ("LoongArch: BPF: Support sign-extension mov instructions")
Acked-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/net/bpf_jit.c

@@ -480,10 +480,12 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool ext
 		case 8:
 			move_reg(ctx, t1, src);
 			emit_insn(ctx, extwb, dst, t1);
+			emit_zext_32(ctx, dst, is32);
 			break;
 		case 16:
 			move_reg(ctx, t1, src);
 			emit_insn(ctx, extwh, dst, t1);
+			emit_zext_32(ctx, dst, is32);
 			break;
 		case 32:
 			emit_insn(ctx, addw, dst, src, LOONGARCH_GPR_ZERO);