bpf: don't accept cgroup local storage with zero value size

Explicitly forbid creating cgroup local storage maps with zero value
size, as it makes no sense and might even cause a panic.

Reported-by:  <syzbot+18628320d3b14a5c459c@syzkaller.appspotmail.com>
Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>