s390/bpf: Fix bpf_arch_text_poke() with new_addr == NULL again

In the Linux kernel, the following vulnerability has been resolved: s390/bpf: Fix bpf_arch_text_poke() with new_addr == NULL again Commit 7ded842b356d ("s390/bpf: Fix bpf_plt pointer arithmetic") has accidentally removed the critical piece of commit c730fce7c70c ("s390/bpf: Fix bpf_arch_text_poke() with new_addr == NULL"), causing intermittent kernel panics in e.g. perf's on_switch() prog to reappear. Restore the fix and add a comment.