bpf: Remove bpf_cpumask_kptr_get() kfunc

bpf: Remove bpf_cpumask_kptr_get() kfunc

Now that struct bpf_cpumask is RCU safe, there's no need for this kfunc.
Rather than doing the following:

private(MASK) static struct bpf_cpumask __kptr *global;

int BPF_PROG(prog, s32 cpu, ...)
{
	struct bpf_cpumask *cpumask;

	bpf_rcu_read_lock();
	cpumask = bpf_cpumask_kptr_get(&global);
	if (!cpumask) {
		bpf_rcu_read_unlock();
		return -1;
	}
	bpf_cpumask_setall(cpumask);
	...
	bpf_cpumask_release(cpumask);
	bpf_rcu_read_unlock();
}

Programs can instead simply do (assume same global cpumask):

int BPF_PROG(prog, ...)
{
	struct bpf_cpumask *cpumask;

	bpf_rcu_read_lock();
	cpumask = global;
	if (!cpumask) {
		bpf_rcu_read_unlock();
		return -1;
	}
	bpf_cpumask_setall(cpumask);
	...
	bpf_rcu_read_unlock();
}

In other words, no extra atomic acquire / release, and less boilerplate
code.

This patch removes both the kfunc, as well as its selftests and
documentation.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230316054028.88924-5-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>