selftests/bpf: fix memory leak of lsm_cgroup

selftests/bpf: fix memory leak of lsm_cgroup

kmemleak reports this issue:

unreferenced object 0xffff88810b7835c0 (size 32):
  comm "test_progs", pid 270, jiffies 4294969007 (age 1621.315s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    03 00 00 00 03 00 00 00 0f 00 00 00 00 00 00 00  ................
  backtrace:
    [<00000000376cdeab>] kmalloc_trace+0x27/0x110
    [<000000003bcdb3b6>] selinux_sk_alloc_security+0x66/0x110
    [<000000003959008f>] security_sk_alloc+0x47/0x80
    [<00000000e7bc6668>] sk_prot_alloc+0xbd/0x1a0
    [<0000000002d6343a>] sk_alloc+0x3b/0x940
    [<000000009812a46d>] unix_create1+0x8f/0x3d0
    [<000000005ed0976b>] unix_create+0xa1/0x150
    [<0000000086a1d27f>] __sock_create+0x233/0x4a0
    [<00000000cffe3a73>] __sys_socket_create.part.0+0xaa/0x110
    [<0000000007c63f20>] __sys_socket+0x49/0xf0
    [<00000000b08753c8>] __x64_sys_socket+0x42/0x50
    [<00000000b56e26b3>] do_syscall_64+0x3b/0x90
    [<000000009b4871b8>] entry_SYSCALL_64_after_hwframe+0x63/0xcd

The issue occurs in the following scenarios:

unix_create1()
  sk_alloc()
    sk_prot_alloc()
      security_sk_alloc()
        call_int_hook()
          hlist_for_each_entry()
            entry1->hook.sk_alloc_security
            <-- selinux_sk_alloc_security() succeeded,
            <-- sk->security alloced here.
            entry2->hook.sk_alloc_security
            <-- bpf_lsm_sk_alloc_security() failed
      goto out_free;
        ...    <-- the sk->security not freed, memleak

The core problem is that the LSM is not yet fully stacked (work is
actively going on in this space) which means that some LSM hooks do
not support multiple LSMs at the same time. To fix, skip the
"EPERM" test when it runs in the environments that already have
non-bpf lsms installed

Fixes: dca85aac8895 ("selftests/bpf: lsm_cgroup functional test")
Signed-off-by: Wang Yufen <wangyufen@huawei.com>
Cc: Stanislav Fomichev <sdf@google.com>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/1668482980-16163-1-git-send-email-wangyufen@huawei.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>