s390/bpf: Use direct calls and jumps where possible

After the V!=R rework (commit c98d2eca ("s390/mm: Uncouple physical
vs virtual address spaces")), all kernel code and related data are
allocated within a 4G region, making it possible to use relative
addressing in BPF code more extensively.

Convert as many indirect calls and jumps to direct calls as possible,
namely:

* BPF_CALL
* __bpf_tramp_enter()
* __bpf_tramp_exit()
* __bpf_prog_enter()
* __bpf_prog_exit()
* fentry
* fmod_ret
* fexit
* BPF_TRAMP_F_CALL_ORIG without BPF_TRAMP_F_ORIG_STACK
* Trampoline returns without BPF_TRAMP_F_SKIP_FRAME and
  BPF_TRAMP_F_ORIG_STACK

The following indirect calls and jumps remain:

* Prog returns
* Trampoline returns with BPF_TRAMP_F_SKIP_FRAME or
  BPF_TRAMP_F_ORIG_STACK
* BPF_TAIL_CALL
* BPF_TRAMP_F_CALL_ORIG with BPF_TRAMP_F_ORIG_STACK

As a result, only one usage of call_r1() remains, so inline it.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20250819102116.252203-1-iii@linux.ibm.com