Merge branch 'bpf_get_current_cgroup_id'

Yonghong Song says:

====================
bpf has been used extensively for tracing. For example, bcc
contains an almost full set of bpf-based tools to trace kernel
and user functions/events. Most tracing tools are currently
either filtered based on pid or system-wide.

Containers have been used quite extensively in industry and
cgroup is often used together to provide resource isolation
and protection. Several processes may run inside the same
container. It is often desirable to get container-level tracing
results as well, e.g. syscall count, function count, I/O
activity, etc.

This patch implements a new helper, bpf_get_current_cgroup_id(),
which will return cgroup id based on the cgroup within which
the current task is running.

Patch #1 implements the new helper in the kernel.
Patch #2 syncs the uapi bpf.h header and helper between tools
and kernel.
Patch #3 shows how to get the same cgroup id in user space,
so a filter or policy could be configgured in the bpf program
based on current task cgroup.

Changelog:
  v1 -> v2:
     . rebase to resolve merge conflict with latest bpf-next.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>