s390/bpf: Fix unwinding past the trampoline

When functions called by the trampoline panic, the backtrace that is
printed stops at the trampoline, because the trampoline does not store
its caller's frame address (backchain) on stack; it also stores the
return address at a wrong location.

Store both the same way as is already done for the regular eBPF programs.

Fixes: 528eb2cb ("s390/bpf: Implement arch_prepare_bpf_trampoline()")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20231010203512.385819-3-iii@linux.ibm.com