bpf: Annotate implicit fall through in cgroup_dev_func_proto

There is a plan to build the kernel with -Wimplicit-fallthrough
and this place in the code produced a warnings (W=1).

This commit removes the following warning:

  kernel/bpf/cgroup.c:719:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
Signed-off-by: NMathieu Malaterre <malat@debian.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

kernel/bpf/cgroup.c

@@ -718,6 +718,7 @@ cgroup_dev_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 	case BPF_FUNC_trace_printk:
 		if (capable(CAP_SYS_ADMIN))
 			return bpf_get_trace_printk_proto();
+		/* fall through */
 	default:
 		return NULL;
 	}