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: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>