sched/deadline: Fix warning in migrate_enable for boosted tasks

In the Linux kernel, the following vulnerability has been resolved:

sched/deadline: Fix warning in migrate_enable for boosted tasks

When running the following command:

while true; do
stress-ng --cyclic 30 --timeout 30s --minimize --quiet
done

a warning is eventually triggered:

WARNING: CPU: 43 PID: 2848 at kernel/sched/deadline.c:794
setup_new_dl_entity+0x13e/0x180
...
Call Trace:
<TASK>
? show_trace_log_lvl+0x1c4/0x2df
? enqueue_dl_entity+0x631/0x6e0
? setup_new_dl_entity+0x13e/0x180
? __warn+0x7e/0xd0
? report_bug+0x11a/0x1a0
? handle_bug+0x3c/0x70
? exc_invalid_op+0x14/0x70
? asm_exc_invalid_op+0x16/0x20
enqueue_dl_entity+0x631/0x6e0
enqueue_task_dl+0x7d/0x120
__do_set_cpus_allowed+0xe3/0x280
__set_cpus_allowed_ptr_locked+0x140/0x1d0
__set_cpus_allowed_ptr+0x54/0xa0
migrate_enable+0x7e/0x150
rt_spin_unlock+0x1c/0x90
group_send_sig_info+0xf7/0x1a0
? kill_pid_info+0x1f/0x1d0
kill_pid_info+0x78/0x1d0
kill_proc_info+0x5b/0x110
__x64_sys_kill+0x93/0xc0
do_syscall_64+0x5c/0xf0
entry_SYSCALL_64_after_hwframe+0x6e/0x76
RIP: 0033:0x7f0dab31f92b

This warning occurs because set_cpus_allowed dequeues and enqueues tasks
with the ENQUEUE_RESTORE flag set. If the task is boosted, the warning
is triggered. A boosted task already had its parameters set by
rt_mutex_setprio, and a new call to setup_new_dl_entity is unnecessary,
hence the WARN_ON call.

Check if we are requeueing a boosted task and avoid calling
setup_new_dl_entity if that's the case.