sched_ext: Rename pnt_seq to kick_sync

sched_ext: Rename pnt_seq to kick_sync

The pnt_seq field and related infrastructure were originally named for
"pick next task sequence", reflecting their original implementation in
scx_next_task_picked(). However, the sequence counter is now incremented in
both put_prev_task_scx() and pick_task_scx() and its purpose is to
synchronize kick operations via SCX_KICK_WAIT, not specifically to track
pick_next_task events.

Rename to better reflect the actual semantics:
- pnt_seq -> kick_sync
- scx_kick_pseqs -> scx_kick_syncs
- pseqs variables -> ksyncs
- Update comments to refer to "kick_sync sequence" instead of "pick_task
  sequence"

This is a pure renaming with no functional changes.

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