sched_ext: Simplify task state handling

sched_ext: Simplify task state handling

Task states (NONE, INIT, READY, ENABLED) were defined in a separate enum with
unshifted values and then shifted when stored in scx_entity.flags. Simplify by
defining them as pre-shifted values directly in scx_ent_flags and removing the
separate scx_task_state enum. This removes the need for shifting when
reading/writing state values.

scx_get_task_state() now returns the masked flags value directly.
scx_set_task_state() accepts the pre-shifted state value. scx_dump_task()
shifts down for display to maintain readable output.

No functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>