selftests/bpf: fix test_cgroup_storage on s390

test_cgroup_storage fails on s390 with an assertion failure: packets are
dropped when they shouldn't. The problem is that BPF_DW packet count is
accessed as BPF_W with an offset of 0, which is not correct on
big-endian machines.

Since the point of this test is not to verify narrow loads/stores,
simply use BPF_DW when working with packet counts.

Fixes: 68cfa3ac ("selftests/bpf: add a cgroup storage test")
Fixes: 919646d2 ("selftests/bpf: extend the storage test to test per-cpu cgroup storage")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>