bpf/selftests: Test using global cpumask kptr with RCU

bpf/selftests: Test using global cpumask kptr with RCU

Now that struct bpf_cpumask * is considered an RCU-safe type according
to the verifier, we should add tests that validate its common usages.
This patch adds those tests to the cpumask test suite. A subsequent
changes will remove bpf_cpumask_kptr_get(), and will adjust the selftest
and BPF documentation accordingly.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230316054028.88924-4-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>