bpf,docs: Remove references to bpf_cgroup_kptr_get()

----

You may also acquire a reference to a ``struct cgroup`` kptr that's already

stored in a map using bpf_cgroup_kptr_get():

.. kernel-doc:: kernel/bpf/helpers.c

   :identifiers: bpf_cgroup_kptr_get

Here's an example of how it can be used:

.. code-block:: c

	/* struct containing the struct task_struct kptr which is actually stored in the map. */

	struct __cgroups_kfunc_map_value {

		struct cgroup __kptr * cgroup;

	};

	/* The map containing struct __cgroups_kfunc_map_value entries. */

	struct {

		__uint(type, BPF_MAP_TYPE_HASH);

		__type(key, int);

		__type(value, struct __cgroups_kfunc_map_value);

		__uint(max_entries, 1);

	} __cgroups_kfunc_map SEC(".maps");

	/* ... */

	/**

	 * A simple example tracepoint program showing how a

	 * struct cgroup kptr that is stored in a map can

	 * be acquired using the bpf_cgroup_kptr_get() kfunc.

	 */

	 SEC("tp_btf/cgroup_mkdir")

	 int BPF_PROG(cgroup_kptr_get_example, struct cgroup *cgrp, const char *path)

	 {

		struct cgroup *kptr;

		struct __cgroups_kfunc_map_value *v;

		s32 id = cgrp->self.id;

		/* Assume a cgroup kptr was previously stored in the map. */

		v = bpf_map_lookup_elem(&__cgroups_kfunc_map, &id);

		if (!v)

			return -ENOENT;

		/* Acquire a reference to the cgroup kptr that's already stored in the map. */

		kptr = bpf_cgroup_kptr_get(&v->cgroup);

		if (!kptr)

			/* If no cgroup was present in the map, it's because

			 * we're racing with another CPU that removed it with

			 * bpf_kptr_xchg() between the bpf_map_lookup_elem()

			 * above, and our call to bpf_cgroup_kptr_get().

			 * bpf_cgroup_kptr_get() internally safely handles this

			 * race, and will return NULL if the task is no longer

			 * present in the map by the time we invoke the kfunc.

			 */

			return -EBUSY;

		/* Free the reference we just took above. Note that the

		 * original struct cgroup kptr is still in the map. It will

		 * be freed either at a later time if another context deletes

		 * it from the map, or automatically by the BPF subsystem if

		 * it's still present when the map is destroyed.

		 */

		bpf_cgroup_release(kptr);

		return 0;

        }

----

Other kfuncs available for interacting with ``struct cgroup *`` objects are

bpf_cgroup_ancestor() and bpf_cgroup_from_id(), allowing callers to access

the ancestor of a cgroup and find a cgroup by its ID, respectively. Both