sched_ext: Replace tryget_task_struct() with get_task_struct()

sched_ext: Replace tryget_task_struct() with get_task_struct()

The tryget_task_struct() calls in scx_sub_disable(),
scx_root_enable_workfn() and scx_sub_enable_workfn() can never fail at
the points they're invoked:

 - scx_root_enable_workfn() iterates over scx_tasks under scx_tasks_lock
   and rq lock. sched_ext_dead() removes tasks from scx_tasks under the
   same scx_tasks_lock before put_task_struct_rcu_user() runs in
   finish_task_switch(). So any task observed in scx_tasks must have
   usage > 0; put_task_struct_rcu_user() hasn't been called and the
   delayed_put_task_struct() callback that decrements usage cannot have
   been queued.

 - scx_sub_disable() and scx_sub_enable_workfn() iterate via
   css_task_iter, which takes a reference on each task in
   css_task_iter_next() and holds it until the next iter_next() call, so
   usage > 0 is guaranteed by the iter itself.

The actual filter for dead tasks is the SCX_TASK_DEAD check inside
scx_task_iter_next_locked(), not tryget; tryget only fails on zero
usage, a state that can't be reached for tasks visible to these iters.

Commit b7d4b28db7da ("sched_ext: Use SCX_TASK_READY test instead of
tryget_task_struct() during class switch") removed an analogous tryget
in the class-switch loop. Convert the remaining tryget calls to plain
get_task_struct() and update the comment in scx_root_enable_workfn()
that suggested tasks could be observed with zero @usage waiting for an
RCU grace period.

Link: https://lore.kernel.org/all/agCLBxHEUqWIepx8@google.com
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>