CVE-2025-21767: clocksource: Use migrate_disable() to avoid calling get_random_u32() in atomic context

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

clocksource: Use migratedisable() to avoid calling getrandomu32() in atomic context

The following bug report happened with a PREEMPTRT kernel:

BUG: sleeping function called from invalid context at kernel/locking/spinlockrt.c:48 inatomic(): 1, irqsdisabled(): 0, nonblock: 0, pid: 2012, name: kwatchdog preemptcount: 1, expected: 0 RCU nest depth: 0, expected: 0 getrandomu32+0x4f/0x110 clocksourceverifychoosecpus+0xab/0x1a0 clocksourceverifypercpu.part.0+0x6b/0x330 clocksourcewatchdogkthread+0x193/0x1a0

It is due to the fact that clocksourceverifychoosecpus() is invoked with preemption disabled. This function invokes getrandomu32() to obtain random numbers for choosing CPUs. The batchedentropy32 local lock and/or the basecrng.lock spinlock in driver/char/random.c will be acquired during the call. In PREEMPTRT kernel, they are both sleeping locks and so cannot be acquired in atomic context.

Fix this problem by using migratedisable() to allow smpprocessorid() to be reliably used without introducing atomic context. preemptdisable() is then called after clocksourceverifychoosecpus() but before the clocksource measurement is being run to avoid introducing unexpected latency.