sched/uclamp: Fix locking around cpu_util_update_eff()

cpu_cgroup_css_online() calls cpu_util_update_eff() without holding the
uclamp_mutex or rcu_read_lock() like other call sites, which is
a mistake.

The uclamp_mutex is required to protect against concurrent reads and
writes that could update the cgroup hierarchy.

The rcu_read_lock() is required to traverse the cgroup data structures
in cpu_util_update_eff().

Surround the caller with the required locks and add some asserts to
better document the dependency in cpu_util_update_eff().

Fixes: 7226017ad37a ("sched/uclamp: Fix a bug in propagating uclamp value in new cgroups")
Reported-by: Quentin Perret <[email protected]>
Signed-off-by: Qais Yousef <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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