s390/bpf: Add expoline to tail calls

All the indirect jumps in the eBPF JIT already use expolines, except
for the tail call one.

Fixes: de5cb6eb ("s390: use expoline thunks in the BPF JIT")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230129190501.1624747-3-iii@linux.ibm.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

arch/s390/net/bpf_jit_comp.c

@@ -1393,8 +1393,16 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		/* lg %r1,bpf_func(%r1) */
 		EMIT6_DISP_LH(0xe3000000, 0x0004, REG_1, REG_1, REG_0,
 			      offsetof(struct bpf_prog, bpf_func));
+		if (nospec_uses_trampoline()) {
+			jit->seen |= SEEN_FUNC;
+			/* aghi %r1,tail_call_start */
+			EMIT4_IMM(0xa70b0000, REG_1, jit->tail_call_start);
+			/* brcl 0xf,__s390_indirect_jump_r1 */
+			EMIT6_PCREL_RILC(0xc0040000, 0xf, jit->r1_thunk_ip);
+		} else {
 			/* bc 0xf,tail_call_start(%r1) */
 			_EMIT4(0x47f01000 + jit->tail_call_start);
+		}
 		/* out: */
 		if (jit->prg_buf) {
 			*(u16 *)(jit->prg_buf + patch_1_clrj + 2) =