docs: split a C snippet in BPF documentation

A snippet of code was recently marked as C, but it actually contains
both C and a sample output from the kernel verifier (which is not C
code). Sphinx raises a warning as it fails to apply syntax highlighting
for C for that snippet. Let's split the C code from the verifier output.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>

Documentation/bpf.rst

@@ -2138,7 +2138,8 @@ describe some of the differences for the BPF model:
         return 0;
     }
 
-    // On bpf_load_program
+  Corresponding output on ``bpf_load_program()``::
+
     bpf_load_program() err=13
     0: (bf) r6 = r1
     ...