sched/deadline: Fix 'stuck' dl_server

sched/deadline: Fix 'stuck' dl_server

Andrea reported the dl_server getting stuck for him. He tracked it
down to a state where dl_server_start() saw dl_defer_running==1, but
the dl_server's job is no longer valid at the time of
dl_server_start().

In the state diagram this corresponds to [4] D->A (or dl_server_stop()
due to no more runnable tasks) followed by [1], which in case of a
lapsed deadline must then be A->B.

Now our A has dl_defer_running==1, while B demands
dl_defer_running==0, therefore it must get cleared when the CBS wakeup
rules demand a replenish.

Fixes: a110a81c52a9 ("sched/deadline: Deferrable dl server")
Reported-by: Andrea Righi arighi@nvidia.com
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Tested-by: Andrea Righi arighi@nvidia.com
Link: https://lkml.kernel.org/r/20260123161645.2181752-1-arighi@nvidia.com
Link: https://patch.msgid.link/20260130124100.GC1079264@noisy.programming.kicks-ass.net