cgroup/rdma: add rdma.events to track resource limit exhaustion

cgroup/rdma: add rdma.events to track resource limit exhaustion

Add per-device hierarchical event counters to track when RDMA resource
limits are exceeded. The rdma.events file reports max event counts
propagated upward from the cgroup whose limit was hit to all ancestors.

This mirrors the design of pids.events, where events are attributed to
the cgroup that imposed the limit, not necessarily the cgroup where the
allocation was attempted. Userspace can monitor this file via
poll/epoll for real-time notification of resource exhaustion.

Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>