tools/bpf: fix selftest get_cgroup_id_user

Commit f269099a ("tools/bpf: add a selftest for
bpf_get_current_cgroup_id() helper") added a test
for bpf_get_current_cgroup_id() helper. The bpf program
is attached to tracepoint syscalls/sys_enter_nanosleep
and will record the cgroup id if the tracepoint is hit.
The test program creates a cgroup and attachs itself to
this cgroup and expects that the test program process
cgroup id is the same as the cgroup_id retrieved
by the bpf program.

In a light system where no other processes called
nanosleep syscall, the test case can pass.
In a busy system where many different processes can hit
syscalls/sys_enter_nanosleep tracepoint, the cgroup id
recorded by bpf program may not match the test program
process cgroup_id.

This patch fixed an issue by communicating the test program
pid to bpf program. The bpf program only records
cgroup id if the current task pid is the same as
passed-in pid. This ensures that the recorded cgroup_id
is for the cgroup within which the test program resides.

Fixes: f269099a

 ("tools/bpf: add a selftest for bpf_get_current_cgroup_id() helper")
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>