fs/resctrl: Fix UAF from worker threads when domains are removed

In the Linux kernel, the following vulnerability has been resolved:

fs/resctrl: Fix UAF from worker threads when domains are removed

The mbm_handle_overflow() and cqm_handle_limbo() workers read event counters
and may sleep while doing so. They are scheduled via delayed_work embedded in
struct rdt_l3_mon_domain. Architecture allocates and frees these domains from
CPU hotplug callbacks under cpus_write_lock(), and the workers acquire
cpus_read_lock() to keep the domain alive across their access.

A use-after-free can occur when a worker is blocked waiting for
cpus_read_lock() while the hotplug core holds cpus_write_lock(): the
architecture frees the rdt_l3_mon_domain that contains the worker's
work_struct. When the worker unblocks, the container_of() it performs on the
embedded work pointer dereferences freed memory.

Drop cpus_read_lock() from the workers and instead drain pending and in-flight
work synchronously before the architecture can free the domain. Since
architecture offlines the domain under cpus_write_lock() after it has been
unlinked from the RCU list and a grace period has elapsed, no new work can be
scheduled. The cancel only needs to wait out existing work. Drop
rdtgroup_mutex during CPU offline around cancel_delayed_work_sync() so that
a worker waiting on the mutex can complete before re-pinning the work on
a different CPU.

When offlining a CPU the architecture may iterate over resources in any order.
For example, the MBA control domain may be offlined before or after
a corresponding L3 monitor domain. Ensure that resctrl fs cancels the workers
no matter what order the architecture offlines the domains.