s390/bpf: Implement arch_prepare_bpf_trampoline()

arch_prepare_bpf_trampoline() is used for direct attachment of eBPF
programs to various places, bypassing kprobes. It's responsible for
calling a number of eBPF programs before, instead and/or after
whatever they are attached to.

Add a s390x implementation, paying attention to the following:

- Reuse the existing JIT infrastructure, where possible.
- Like the existing JIT, prefer making multiple passes instead of
  backpatching. Currently 2 passes is enough. If literal pool is
  introduced, this needs to be raised to 3. However, at the moment
  adding literal pool only makes the code larger. If branch
  shortening is introduced, the number of passes needs to be
  increased even further.
- Support both regular and ftrace calling conventions, depending on
  the trampoline flags.
- Use expolines for indirect calls.
- Handle the mismatch between the eBPF and the s390x ABIs.
- Sign-extend fmod_ret return values.

invoke_bpf_prog() produces about 120 bytes; it might be possible to
slightly optimize this, but reaching 50 bytes, like on x86_64, looks
unrealistic: just loading cookie, __bpf_prog_enter, bpf_func, insnsi
and __bpf_prog_exit as literals already takes at least 5 * 12 = 60
bytes, and we can't use relative addressing for most of them.
Therefore, lower BPF_MAX_TRAMP_LINKS on s390x.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230129190501.1624747-5-iii@linux.ibm.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -944,7 +944,11 @@ struct btf_func_model {
  * bytes on x86.
  */
 enum {
+#if defined(__s390x__)
+	BPF_MAX_TRAMP_LINKS = 27,
+#else
 	BPF_MAX_TRAMP_LINKS = 38,
+#endif
 };
 
 struct bpf_tramp_links {