s390/bpf: Write back tail call counter for BPF_TRAMP_F_CALL_ORIG

The tailcall_bpf2bpf_hierarchy_fentry test hangs on s390. Its call
graph is as follows:

entry()
subprog_tail()
trampoline()
fentry()
the rest of subprog_tail() # via BPF_TRAMP_F_CALL_ORIG
return to entry()

The problem is that the rest of subprog_tail() increments the tail call
counter, but the trampoline discards the incremented value. This
results in an astronomically large number of tail calls.

Fix by making the trampoline write the incremented tail call counter
back.

Fixes: 528eb2cb87bc ("s390/bpf: Implement arch_prepare_bpf_trampoline()")
Signed-off-by: Ilya Leoshkevich <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

arch/s390/net/bpf_jit_comp.c

···
 		/* stg %r2,retval_off(%r15) */
 		EMIT6_DISP_LH(0xe3000000, 0x0024, REG_2, REG_0, REG_15,
 			      tjit->retval_off);
+		/* mvc tccnt_off(%r15),tail_call_cnt(4,%r15) */
+		_EMIT6(0xd203f000 | tjit->tccnt_off,
+		       0xf000 | offsetof(struct prog_frame, tail_call_cnt));
 
 		im->ip_after_call = jit->prg_buf + jit->prg;