s390/bpf: Use brcl for jumping to exit_ip if necessary

"BPF_MAXINSNS: Maximum possible literals" test causes panic with
bpf_jit_harden = 2. The reason is that BPF_JMP | BPF_EXIT is always
emitted as brc, however, after removal of JITed image size
limitations, brcl might be required.

Fix by using brcl when necessary.

Fixes: 4e9b4a6 ("s390/bpf: Use relative long branches")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200717165326.6786-4-iii@linux.ibm.com

arch/s390/net/bpf_jit_comp.c

@@ -1268,8 +1268,12 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		last = (i == fp->len - 1) ? 1 : 0;
 		if (last)
 			break;
-		/* j <exit> */
-		EMIT4_PCREL(0xa7f40000, jit->exit_ip - jit->prg);
+		if (!is_first_pass(jit) && can_use_rel(jit, jit->exit_ip))
+			/* brc 0xf, <exit> */
+			EMIT4_PCREL_RIC(0xa7040000, 0xf, jit->exit_ip);
+		else
+			/* brcl 0xf, <exit> */
+			EMIT6_PCREL_RILC(0xc0040000, 0xf, jit->exit_ip);
 		break;
 	/*
 	 * Branch relative (number of skipped instructions) to offset on