s390/bpf: Introduce pre- and post- probe functions

Currently probe insns are handled by two "if" statements at the
beginning and at the end of bpf_jit_insn(). The first one needs to be
in sync with the huge insn->code statement that follows it, which was
not a problem so far, since the check is small.

The introduction of arena will make it significantly larger, and it
will no longer be obvious whether it is in sync with the opcode switch.

Move these statements to the new bpf_jit_probe_load_pre() and
bpf_jit_probe_post() functions, and call them only from cases that need
them.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240701234304.14336-5-iii@linux.ibm.com