s390/bpf: Do not write tail call counter into helper and kfunc frames

Only BPF functions make use of the tail call counter; helpers and
kfuncs ignore and most likely also clobber it. Writing it into these
functions' frames is pointless and misleading, so do not do it.

Fixes: dd691e84 ("s390/bpf: Implement bpf_jit_supports_subprog_tailcalls()")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20250813121016.163375-2-iii@linux.ibm.com