bpf: Refactor do_check() if/else into do_check_insn()

Move code ito do_check_insn(), replace
- "continue" with "return CHECK_NEXT_INSN"
- "break" with "return ALL_PATHS_CHECKED"
- "do_print_state = " with "*do_print_state = "
- "goto process_bpf_exit" / fallthrough with "return process_bpf_exit()"