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

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.

Fixes: 7ded842b356d ("s390/bpf: Fix bpf_plt pointer arithmetic")
Cc: stable@vger.kernel.org
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20250716194524.48109-2-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>