bpf: Mark struct bpf_cpumask as rcu protected

bpf: Mark struct bpf_cpumask as rcu protected

struct bpf_cpumask is a BPF-wrapper around the struct cpumask type which
can be instantiated by a BPF program, and then queried as a cpumask in
similar fashion to normal kernel code. The previous patch in this series
makes the type fully RCU safe, so the type can be included in the
rcu_protected_type BTF ID list.

A subsequent patch will remove bpf_cpumask_kptr_get(), as it's no longer
useful now that we can just treat the type as RCU safe by default and do
our own if check.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230316054028.88924-3-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>