ANDROID: cgroup/cpuset: Do not access task_cs(p) outside of RCU

task_cs(p) is protected by RCU, so ensure that we have entered an RCU
read-side critical section before accessing it in guarantee_online_cpus().

This issue was introduced by 4045a05f ("BACKPORT: FROMLIST: cpuset:
Honour task_cpu_possible_mask() in guarantee_online_cpus()") and spotted
during upstream review.

Reported-by: Qais Yousef <qais.yousef@arm.com>
Link: https://lore.kernel.org/r/20210521162524.22cwmrao3df7m4jb@e107158-lin.cambridge.arm.com


Fixes: 4045a05f ("BACKPORT: FROMLIST: cpuset: Honour task_cpu_possible_mask() in guarantee_online_cpus()")
Bug: 178507149
Change-Id: Ia8b8b89b5fcf72eefe9c2667951e24f315176ed5
Signed-off-by: Will Deacon <willdeacon@google.com>