Merge branch 's390-bpf-fix-arch_prepare_bpf_trampoline'

Song Liu says:

====================
s390/bpf: Fix arch_prepare_bpf_trampoline

While working on trampoline, I found s390's arch_prepare_bpf_trampoline
returns 0 on success, which breaks struct_ops. However, the CI doesn't
catch this issue. Turns out test_progs:bpf_tcp_ca doesn't really test
members of a struct_ops are actually called via the trampolines.

1/2 fixes arch_prepare_bpf_trampoline for s390.
2/2 adds a check to test_progs:bpf_tcp_ca to verify bpf_cubic_acked() is
indeed called by the trampoline. Without 1/2, this check would fail on
s390.
====================

Link: https://lore.kernel.org/r/20230919060258.3237176-1-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>