cgroup/cpuset: Prevent NULL pointer access in free_tmpmasks()

cgroup/cpuset: Prevent NULL pointer access in free_tmpmasks()

Commit 5806b3d05165 ("cpuset: decouple tmpmasks and cpumasks freeing in
cgroup") separates out the freeing of tmpmasks into a new free_tmpmask()
helper but removes the NULL pointer check in the process. Unfortunately a
NULL pointer can be passed to free_tmpmasks() in cpuset_handle_hotplug()
if cpuset v1 is active. This can cause segmentation fault and crash
the kernel.

Fix that by adding the NULL pointer check to free_tmpmasks().

Fixes: 5806b3d05165 ("cpuset: decouple tmpmasks and cpumasks freeing in cgroup")
Reported-by: Ashay Jaiswal <quic_ashayj@quicinc.com>
Closes: https://lore.kernel.org/lkml/20250902-cpuset-free-on-condition-v1-1-f46ffab53eac@quicinc.com/
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>