sched/fair: Remove task_group->se pointer array

Now that struct sched_entity is co-located with struct cfs_rq for non-root task
groups, the task_group->se pointer array is redundant. The associated
sched_entity can be loaded directly from the cfs_rq.

This patch performs the access conversion with the helpers:

 - is_root_task_group(tg): checks if a task group is the root task group. It
   compares the task group's address with the global root_task_group variable.

 - tg_se(tg, cpu): retrieves the cfs_rq and returns the address of the
   co-located se. This function checks if tg is the root task group to ensure
   behaving the same of previous tg->se[cpu]. Replaces all accesses that use
   the tg->se[cpu] pointer array with calls to the new tg_se(tg, cpu) accessor.

 - cfs_rq_se(cfs_rq): simplifies access paths like cfs_rq->tg->se[...] to use
   the co-located sched_entity. This function also checks if tg is the root
   task group to ensure same behavior.

Since tg_se is not in very hot code paths, and the branch is a register
comparison with an immediate value (`&root_task_group`), the performance impact
is expected to be negligible.

Signed-off-by: Zecheng Li <zecheng@google.com>
Signed-off-by: Zecheng Li <zli94@ncsu.edu>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Reviewed-by: Josh Don <joshdon@google.com>
Link: https://patch.msgid.link/20260522141623.600235-3-zli94@ncsu.edu

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