sched_ext: Decouple locks in scx_ops_enable()

The enable path uses three big locks - scx_fork_rwsem, scx_cgroup_rwsem and
cpus_read_lock. Currently, the locks are grabbed together which is prone to
locking order problems.

For example, currently, there is a possible deadlock involving
scx_fork_rwsem and cpus_read_lock. cpus_read_lock has to nest inside
scx_fork_rwsem due to locking order existing in other subsystems. However,
there exists a dependency in the other direction during hotplug if hotplug
needs to fork a new task, which happens in some cases. This leads to the
following deadlock:

       scx_ops_enable()                               hotplug

                                          percpu_down_write(&cpu_hotplug_lock)
   percpu_down_write(&scx_fork_rwsem)
   block on cpu_hotplug_lock
                                          kthread_create() waits for kthreadd
					  kthreadd blocks on scx_fork_rwsem

Note that this doesn't trigger lockdep because the hotplug side dependency
bounces through kthreadd.

With the preceding scx_cgroup_enabled change, this can be solved by
decoupling cpus_read_lock, which is needed for static_key manipulations,
from the other two locks.

- Move the first block of static_key manipulations outside of scx_fork_rwsem
  and scx_cgroup_rwsem. This is now safe with the preceding
  scx_cgroup_enabled change.

- Drop scx_cgroup_rwsem and scx_fork_rwsem between the two task iteration
  blocks so that __scx_ops_enabled static_key enabling is outside the two
  rwsems.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-tested-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Link: http://lkml.kernel.org/r/8cd0ec0c4c7c1bc0119e61fbef0bee9d5e24022d.camel@linux.ibm.com

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