bpf, x86: Adjust arch_prepare_bpf_trampoline return value

bpf, x86: Adjust arch_prepare_bpf_trampoline return value

x86's implementation of arch_prepare_bpf_trampoline() requires
BPF_INSN_SAFETY buffer space between end of program and image_end. OTOH,
the return value does not include BPF_INSN_SAFETY. This doesn't cause any
real issue at the moment. However, "image" of size retval is not enough for
arch_prepare_bpf_trampoline(). This will cause confusion when we introduce
a new helper arch_bpf_trampoline_size(). To avoid future confusion, adjust
the return value to include BPF_INSN_SAFETY.

Signed-off-by: Song Liu <song@kernel.org>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20231206224054.492250-5-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>