Merge branch 'Make struct bpf_cpumask RCU safe'

David Vernet says:

====================

The struct bpf_cpumask type is currently not RCU safe. It uses the
bpf_mem_cache_{alloc,free}() APIs to allocate and release cpumasks, and
those allocations may be reused before an RCU grace period has elapsed.
We want to be able to enable using this pattern in BPF programs:

private(MASK) static struct bpf_cpumask __kptr *global;

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, to be able to pass a kptr to KF_RCU bpf_cpumask kfuncs
without requiring the acquisition and release of refcounts using
bpf_cpumask_kptr_get(). This patchset enables this by making the struct
bpf_cpumask type RCU safe, and removing the bpf_cpumask_kptr_get()
function.
---
v1: https://lore.kernel.org/all/20230316014122.678082-2-void@manifault.com/

Changelog:
----------
v1 -> v2:
- Add doxygen comment for new @rcu field in struct bpf_cpumask.
====================

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