sched_ext: Replace SCX_TASK_BAL_KEEP with SCX_RQ_BAL_KEEP

SCX_TASK_BAL_KEEP is used by balance_one() to tell pick_next_task_scx() to
keep running the current task. It's not really a task property. Replace it
with SCX_RQ_BAL_KEEP which resides in rq->scx.flags and is a better fit for
the usage. Also, the existing clearing rule is unnecessarily strict and
makes it difficult to use with core-sched. Just clear it on entry to
balance_one().

Signed-off-by: Tejun Heo <tj@kernel.org>