bpf, test, cgroup: Use sk_{alloc,free} for test cases

BPF test infra has some hacks in place which kzalloc() a socket and perform
minimum init via sock_net_set() and sock_init_data(). As a result, the sk's
skcd->cgroup is NULL since it didn't go through proper initialization as it
would have been the case from sk_alloc(). Rather than re-adding a NULL test
in sock_cgroup_ptr() just for this, use sk_{alloc,free}() pair for the test
socket. The latter also allows to get rid of the bpf_sk_storage_free() special
case.

Fixes: 8520e224 ("bpf, cgroups: Fix cgroup v2 fallback on v1/v2 mixed mode")
Fixes: b7a1848e ("bpf: add BPF_PROG_TEST_RUN support for flow dissector")
Fixes: 2cb494a3 ("bpf: add tests for direct packet access from CGROUP_SKB")
Reported-by:  <syzbot+664b58e9a40fbb2cec71@syzkaller.appspotmail.com>
Reported-by:  <syzbot+33f36d0754d4c5c0e102@syzkaller.appspotmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Tested-by:  <syzbot+664b58e9a40fbb2cec71@syzkaller.appspotmail.com>
Tested-by:  <syzbot+33f36d0754d4c5c0e102@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/bpf/20210927123921.21535-2-daniel@iogearbox.net