sched/deadline: Fix server stopping with runnable tasks

sched/deadline: Fix server stopping with runnable tasks

The deadline server can currently stop due to idle although fair tasks
are runnable. This happens essentially when:

 * the server is set to idle, a task wakes up, the server stops
 * a task wakes up, the server sets itself to idle and stops right away

Address both cases by clearing the server idle flag whenever a fair task
wakes up and accounting also for pending tasks in the definition of idle.

Fixes: f5a538c07df2 ("sched/deadline: Fix dl_server stop condition")
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260113085159.114226-3-gmonaco@redhat.com