s390/bpf: Fix 64-bit subtraction of the -0x80000000 constant

The JIT uses agfi for subtracting constants, but -(-0x80000000) cannot
be represented as a 32-bit signed binary integer. Fix by using algfi in
this particular case.

Reported-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Fixes: 0546231057 ("s390/bpf: Add s390x eBPF JIT compiler backend")
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/net/bpf_jit_comp.c

@@ -794,8 +794,13 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 	case BPF_ALU64 | BPF_SUB | BPF_K: /* dst = dst - imm */
 		if (!imm)
 			break;
+		if (imm == -0x80000000) {
+			/* algfi %dst,0x80000000 */
+			EMIT6_IMM(0xc20a0000, dst_reg, 0x80000000);
+		} else {
 			/* agfi %dst,-imm */
 			EMIT6_IMM(0xc2080000, dst_reg, -imm);
+		}
 		break;
 	/*
 	 * BPF_MUL