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.